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V3_chap02 — Shell Command Language

2. Shell Command Language

This chapter contains the definition of the Shell Command Language.

2.1 Shell Introduction

The shell is a command language interpreter. This chapter describes the syntax of that command language as it is used by the sh(1) utility and the system(3) and popen(3) functions defined in the System Interfaces volume of POSIX.1-2008.

The shell operates according to the following general overview of operations. The specific details are included in the cited sections of this chapter.

1. The shell reads its input from a file (see sh(1)), from the -c option or from the system(3) and popen(3) functions defined in the System Interfaces volume of POSIX.1-2008. If the first line of a file of shell commands starts with the characters "#!", the results are unspecified.
2. The shell breaks the input into tokens: words and operators; see Token Recognition.
3. The shell parses the input into simple commands (see Simple Commands) and compound commands (see Compound Commands).
4. The shell performs various expansions (separately) on different parts of each command, resulting in a list of pathnames and fields to be treated as a command and arguments; see wordexp(3).
5. The shell performs redirection (see Redirection) and removes redirection operators and their operands from the parameter list.
6. The shell executes a function (see Function Definition Command), built-in (see Special Built-In Utilities), executable file, or script, giving the names of the arguments as positional parameters numbered 1 to n, and the name of the command (or in the case of a function within a script, the name of the script) as the positional parameter numbered 0 (see Command Search and Execution).
7. The shell optionally waits for the command to complete and collects the exit status (see Exit Status for Commands).

2.2 Quoting

Quoting is used to remove the special meaning of certain characters or words to the shell. Quoting can be used to preserve the literal meaning of the special characters in the next paragraph, prevent reserved words from being recognized as such, and prevent parameter expansion and command substitution within here-document processing (see Here-Document).

The application shall quote the following characters if they are to represent themselves:

|  &  ;  <  >  (  )  $  `  \  "  '  <space>  <tab>  <newline>

and the following may need to be quoted under certain circumstances. That is, these characters may be special depending on conditions described elsewhere in this volume of POSIX.1-2008:

*   ?   [   #   ˜   =   %

The various quoting mechanisms are the escape character, single-quotes, and double-quotes. The here-document represents another form of quoting; see Here-Document.

2.2.1 Escape Character (Backslash)

A <backslash> that is not quoted shall preserve the literal value of the following character, with the exception of a <newline>. If a <newline> follows the <backslash>, the shell shall interpret this as line continuation. The <backslash> and <newline> shall be removed before splitting the input into tokens. Since the escaped <newline> is removed entirely from the input and is not replaced by any white space, it cannot serve as a token separator.

2.2.2 Single-Quotes

Enclosing characters in single-quotes ( ‘’ ) shall preserve the literal value of each character within the single-quotes. A single-quote cannot occur within single-quotes.

2.2.3 Double-Quotes

Enclosing characters in double-quotes ( "" ) shall preserve the literal value of all characters within the double-quotes, with the exception of the characters backquote, <dollar-sign>, and <backslash>, as follows:

$

The <dollar-sign> shall retain its special meaning introducing parameter expansion (see Parameter Expansion ), a form of command substitution (see Command Substitution ), and arithmetic expansion (see Arithmetic Expansion ).

The input characters within the quoted string that are also enclosed between "$(" and the matching ‘’) shall not be affected by the double-quotes, but rather shall define that command whose output replaces the "$(...)" when the word is expanded. The tokenizing rules in Token Recognition, not including the alias substitutions in Alias Substitution, shall be applied recursively to find the matching ‘’).

Within the string of characters from an enclosed "${" to the matching ‘}’, an even number of unescaped double-quotes or single-quotes, if any, shall occur. A preceding <backslash> character shall be used to escape a literal ‘{’ or ‘}’. The rule in Parameter Expansion shall be used to determine the matching ‘}’.

`

The backquote shall retain its special meaning introducing the other form of command substitution (see Command Substitution ). The portion of the quoted string from the initial backquote and the characters up to the next backquote that is not preceded by a <backslash>, having escape characters removed, defines that command whose output replaces "`...`" when the word is expanded. Either of the following cases produces undefined results:

• A single-quoted or double-quoted string that begins, but does not end, within the "`...`" sequence
• A "`...`" sequence that begins, but does not end, within the same double-quoted string

\

The <backslash> shall retain its special meaning as an escape character (see Escape Character (Backslash) ) only when followed by one of the following characters when considered special:

$   `   "   \   <newline>

The application shall ensure that a double-quote is preceded by a <backslash> to be included within double-quotes. The parameter ‘@’ has special meaning inside double-quotes and is described in Special Parameters .

2.3 Token Recognition

The shell shall read its input in terms of lines. (For details about how the shell reads its input, see the description of sh(1).) The input lines can be of unlimited length. These lines shall be parsed using two major modes: ordinary token recognition and processing of here-documents.

When an io_here token has been recognized by the grammar (see Shell Grammar), one or more of the subsequent lines immediately following the next NEWLINE token form the body of one or more here-documents and shall be parsed according to the rules of Here-Document.

When it is not processing an io_here, the shell shall break its input into tokens by applying the first applicable rule below to the next character in its input. The token shall be from the current position in the input until a token is delimited according to one of the rules below; the characters forming the token are exactly those in the input, including any quoting characters. If it is indicated that a token is delimited, and no characters have been included in a token, processing shall continue until an actual token is delimited.

1. If the end of input is recognized, the current token (if any) shall be delimited.
2. If the previous character was used as part of an operator and the current character is not quoted and can be used with the previous characters to form an operator, it shall be used as part of that (operator) token.
3. If the previous character was used as part of an operator and the current character cannot be used with the previous characters to form an operator, the operator containing the previous character shall be delimited.
4. If the current character is <backslash>, single-quote, or double-quote and it is not quoted, it shall affect quoting for subsequent characters up to the end of the quoted text. The rules for quoting are as described in Quoting . During token recognition no substitutions shall be actually performed, and the result token shall contain exactly the characters that appear in the input (except for <newline> joining), unmodified, including any embedded or enclosing quotes or substitution operators, between the <quotation-mark> and the end of the quoted text. The token shall not be delimited by the end of the quoted field.
5. If the current character is an unquoted ‘$’ or ‘`’, the shell shall identify the start of any candidates for parameter expansion ( Parameter Expansion), command substitution ( Command Substitution), or arithmetic expansion ( Arithmetic Expansion) from their introductory unquoted character sequences: ‘$’ or "${", "$(" or ‘`’, and "$((", respectively. The shell shall read sufficient input to determine the end of the unit to be expanded (as explained in the cited sections). While processing the characters, if instances of expansions or quoting are found nested within the substitution, the shell shall recursively process them in the manner specified for the construct that is found. The characters found from the beginning of the substitution to its end, allowing for any recursion necessary to recognize embedded constructs, shall be included unmodified in the result token, including any embedded or enclosing substitution operators or quotes. The token shall not be delimited by the end of the substitution.
6. If the current character is not quoted and can be used as the first character of a new operator, the current token (if any) shall be delimited. The current character shall be used as the beginning of the next (operator) token.
7. If the current character is an unquoted <blank>, any token containing the previous character is delimited and the current character shall be discarded.
8. If the previous character was part of a word, the current character shall be appended to that word.
9. If the current character is a ‘#’, it and all subsequent characters up to, but excluding, the next <newline> shall be discarded as a comment. The <newline> that ends the line is not considered part of the comment.
10. The current character is used as the start of a new word.

Once a token is delimited, it is categorized as required by the grammar in Shell Grammar.

2.3.1 Alias Substitution

After a token has been delimited, but before applying the grammatical rules in Shell Grammar, a resulting word that is identified to be the command name word of a simple command shall be examined to determine whether it is an unquoted, valid alias name. However, reserved words in correct grammatical context shall not be candidates for alias substitution. A valid alias name (see XBD V1_chap03(7)) shall be one that has been defined by the alias(1) utility and not subsequently undefined using unalias(1). Implementations also may provide predefined valid aliases that are in effect when the shell is invoked. To prevent infinite loops in recursive aliasing, if the shell is not currently processing an alias of the same name, the word shall be replaced by the value of the alias; otherwise, it shall not be replaced.

If the value of the alias replacing the word ends in a <blank>, the shell shall check the next command word for alias substitution; this process shall continue until a word is found that is not a valid alias or an alias value does not end in a <blank>.

When used as specified by this volume of POSIX.1-2008, alias definitions shall not be inherited by separate invocations of the shell or by the utility execution environments invoked by the shell; see Shell Execution Environment.

2.4 Reserved Words

Reserved words are words that have special meaning to the shell; see Shell Commands. The following words shall be recognized as reserved words:

This recognition shall only occur when none of the characters is quoted and when the word is used as:

• The first word of a command
• The first word following one of the reserved words other than case, for, or in
• The third word in a case command (only in is valid in this case)
• The third word in a for command (only in and do are valid in this case)

See the grammar in Shell Grammar.

The following words may be recognized as reserved words on some implementations (when none of the characters are quoted), causing unspecified results:

Words that are the concatenation of a name and a <colon> ( ‘’: ) are reserved; their use produces unspecified results.

2.5 Parameters and Variables

A parameter can be denoted by a name, a number, or one of the special characters listed in Special Parameters. A variable is a parameter denoted by a name.

A parameter is set if it has an assigned value (null is a valid value). Once a variable is set, it can only be unset by using the unset special built-in command.

2.5.1 Positional Parameters

A positional parameter is a parameter denoted by the decimal value represented by one or more digits, other than the single digit 0. The digits denoting the positional parameters shall always be interpreted as a decimal value, even if there is a leading zero. When a positional parameter with more than one digit is specified, the application shall enclose the digits in braces (see Parameter Expansion). Positional parameters are initially assigned when the shell is invoked (see sh(1)), temporarily replaced when a shell function is invoked (see Function Definition Command), and can be reassigned with the set special built-in command.

2.5.2 Special Parameters

Listed below are the special parameters and the values to which they shall expand. Only the values of the special parameters are listed; see wordexp(3) for a detailed summary of all the stages involved in expanding words.

@

Expands to the positional parameters, starting from one, initially producing one field for each positional parameter that is set. When the expansion occurs in a context where field splitting will be performed, any empty fields may be discarded and each of the non-empty fields shall be further split as described in Field Splitting . When the expansion occurs within double-quotes, the behavior is unspecified unless one of the following is true:

• Field splitting as described in Field Splitting would be performed if the expansion were not within double-quotes (regardless of whether field splitting would have any effect; for example, if IFS is null).
• The double-quotes are within the word of a ${ parameter: - word } or a ${ parameter: + word } expansion (with or without the <colon>; see Parameter Expansion) which would have been subject to field splitting if parameter had been expanded instead of word.

If one of these conditions is true, the initial fields shall be retained as separate fields, except that if the parameter being expanded was embedded within a word, the first field shall be joined with the beginning part of the original word and the last field shall be joined with the end part of the original word. In all other contexts the results of the expansion are unspecified. If there are no positional parameters, the expansion of ‘@’ shall generate zero fields, even when ‘@’ is within double-quotes; however, if the expansion is embedded within a word which contains one or more other parts that expand to a quoted null string, these null string(s) shall still produce an empty field, except that if the other parts are all within the same double-quotes as the ‘@’, it is unspecified whether the result is zero fields or one empty field.

*

Expands to the positional parameters, starting from one, initially producing one field for each positional parameter that is set. When the expansion occurs in a context where field splitting will be performed, any empty fields may be discarded and each of the non-empty fields shall be further split as described in Field Splitting . When the expansion occurs in a context where field splitting will not be performed, the initial fields shall be joined to form a single field with the value of each parameter separated by the first character of the IFS variable if IFS contains at least one character, or separated by a <space> if IFS is unset, or with no separation if IFS is set to a null string.

#

Expands to the decimal number of positional parameters. The command name (parameter 0) shall not be counted in the number given by '#' because it is a special parameter, not a positional parameter.

?

Expands to the decimal exit status of the most recent pipeline (see Pipelines ).

-

(Hyphen.) Expands to the current option flags (the single-letter option names concatenated into a string) as specified on invocation, by the set special built-in command, or implicitly by the shell.

$

Expands to the decimal process ID of the invoked shell. In a subshell (see Shell Execution Environment ), '$' shall expand to the same value as that of the current shell.

!

Expands to the decimal process ID of the most recent background command (see Lists ) executed from the current shell. (For example, background commands executed from subshells do not affect the value of "$!" in the current shell environment.) For a pipeline, the process ID is that of the last command in the pipeline.

0

(Zero.) Expands to the name of the shell or shell script. See sh for a detailed description of how this name is derived.

See the description of the IFS variable in Shell Variables.

2.5.3 Shell Variables

Variables shall be initialized from the environment (as defined by XBD V1_chap08(7) and the exec function in the System Interfaces volume of POSIX.1-2008) and can be given new values with variable assignment commands. If a variable is initialized from the environment, it shall be marked for export immediately; see the export special built-in. New variables can be defined and initialized with variable assignments, with the read(1) or getopts(1) utilities, with the name parameter in a for loop, with the ${ name = word } expansion, or with other mechanisms provided as implementation extensions.

The following variables shall affect the execution of the shell:

ENV

[UP] The processing of the ENV shell variable shall be supported if the system supports the User Portability Utilities option.

This variable, when and only when an interactive shell is invoked, shall be subjected to parameter expansion (see Parameter Expansion) by the shell and the resulting value shall be used as a pathname of a file containing shell commands to execute in the current environment. The file need not be executable. If the expanded value of ENV is not an absolute pathname, the results are unspecified. ENV shall be ignored if the user's real and effective user IDs or real and effective group IDs are different.

HOME

The pathname of the user's home directory. The contents of HOME are used in tilde expansion (see Tilde Expansion ).

IFS

A string treated as a list of characters that is used for field splitting, expansion of the '*' special parameter, and to split lines into fields with the read utility. If the value of IFS includes any bytes that do not form part of a valid character, the results of field splitting, expansion of '*' , and use of the read utility are unspecified.

If IFS is not set, it shall behave as normal for an unset variable, except that field splitting by the shell and line splitting by the read(1) utility shall be performed as if the value of IFS is <space> <tab> <newline>; see Field Splitting.

The shell shall set IFS to <space> <tab> <newline> when it is invoked.

LANG

Provide a default value for the internationalization variables that are unset or null. (See XBD Internationalization Variables for the precedence of internationalization variables used to determine the values of locale categories.)

LC_ALL

The value of this variable overrides the LC_* variables and LANG, as described in XBD Environment Variables .

LC_COLLATE

Determine the behavior of range expressions, equivalence classes, and multi-character collating elements within pattern matching.

LC_CTYPE

Determine the interpretation of sequences of bytes of text data as characters (for example, single-byte as opposed to multi-byte characters), which characters are defined as letters (character class alpha ) and <blank> characters (character class blank ), and the behavior of character classes within pattern matching. Changing the value of LC_CTYPE after the shell has started shall not affect the lexical processing of shell commands in the current shell execution environment or its subshells. Invoking a shell script or performing exec sh subjects the new shell to the changes in LC_CTYPE.

LC_MESSAGES

Determine the language in which messages should be written.

LINENO

Set by the shell to a decimal number representing the current sequential line number (numbered starting with 1) within a script or function before it executes each command. If the user unsets or resets LINENO, the variable may lose its special meaning for the life of the shell. If the shell is not currently executing a script or function, the value of LINENO is unspecified. This volume of POSIX.1-2008 specifies the effects of the variable only for systems supporting the User Portability Utilities option.

NLSPATH

[XSI] Determine the location of message catalogs for the processing of LC_MESSAGES.

PATH

A string formatted as described in XBD Environment Variables , used to effect command interpretation; see Command Search and Execution .

PPID

Set by the shell to the decimal value of its parent process ID during initialization of the shell. In a subshell (see Shell Execution Environment ), PPID shall be set to the same value as that of the parent of the current shell. For example, echo $ PPID and ( echo $ PPID) would produce the same value.

PS1

Each time an interactive shell is ready to read a command, the value of this variable shall be subjected to parameter expansion and written to standard error. The default value shall be "$ " . For users who have specific additional implementation-defined privileges, the default may be another, implementation-defined value. The shell shall replace each instance of the character '!' in PS1 with the history file number of the next command to be typed. Escaping the '!' with another '!' (that is, "!!" ) shall place the literal character '!' in the prompt. This volume of POSIX.1-2008 specifies the effects of the variable only for systems supporting the User Portability Utilities option.

PS2

Each time the user enters a <newline> prior to completing a command line in an interactive shell, the value of this variable shall be subjected to parameter expansion and written to standard error. The default value is "> " . This volume of POSIX.1-2008 specifies the effects of the variable only for systems supporting the User Portability Utilities option.

PS4

When an execution trace ( set -x ) is being performed in an interactive shell, before each line in the execution trace, the value of this variable shall be subjected to parameter expansion and written to standard error. The default value is "+ " . This volume of POSIX.1-2008 specifies the effects of the variable only for systems supporting the User Portability Utilities option.

PWD

Set by the shell and by the cd utility. In the shell the value shall be initialized from the environment as follows. If a value for PWD is passed to the shell in the environment when it is executed, the value is an absolute pathname of the current working directory that is no longer than {PATH_MAX} bytes including the terminating null byte, and the value does not contain any components that are dot or dot-dot, then the shell shall set PWD to the value from the environment. Otherwise, if a value for PWD is passed to the shell in the environment when it is executed, the value is an absolute pathname of the current working directory, and the value does not contain any components that are dot or dot-dot, then it is unspecified whether the shell sets PWD to the value from the environment or sets PWD to the pathname that would be output by pwd -P . Otherwise, the sh utility sets PWD to the pathname that would be output by pwd -P . In cases where PWD is set to the value from the environment, the value can contain components that refer to files of type symbolic link. In cases where PWD is set to the pathname that would be output by pwd -P , if there is insufficient permission on the current working directory, or on any parent of that directory, to determine what that pathname would be, the value of PWD is unspecified. Assignments to this variable may be ignored. If an application sets or unsets the value of PWD, the behaviors of the cd and pwd utilities are unspecified.

2.6 Word Expansions

This section describes the various expansions that are performed on words. Not all expansions are performed on every word, as explained in the following sections.

Tilde expansions, parameter expansions, command substitutions, arithmetic expansions, and quote removals that occur within a single word expand to a single field. It is only field splitting or pathname expansion that can create multiple fields from a single word. The single exception to this rule is the expansion of the special parameter ‘@’ within double-quotes, as described in Special Parameters.

The order of word expansion shall be as follows:

1. Tilde expansion (see Tilde Expansion), parameter expansion (see Parameter Expansion), command substitution (see Command Substitution), and arithmetic expansion (see Arithmetic Expansion) shall be performed, beginning to end. See item 5 in Token Recognition.
2. Field splitting (see Field Splitting) shall be performed on the portions of the fields generated by step 1, unless IFS is null.
3. Pathname expansion (see Pathname Expansion) shall be performed, unless set -f is in effect.
4. Quote removal (see Quote Removal) shall always be performed last.

The expansions described in this section shall occur in the same shell environment as that in which the command is executed.

If the complete expansion appropriate for a word results in an empty field, that empty field shall be deleted from the list of fields that form the completely expanded command, unless the original word contained single-quote or double-quote characters.

The ‘$’ character is used to introduce parameter expansion, command substitution, or arithmetic evaluation. If an unquoted ‘$’ is followed by a character that is not one of the following:

• A numeric character
• The name of one of the special parameters (see Special Parameters)
• A valid first character of a variable name
• A <left-curly-bracket> ( ‘{’ )
• A <left-parenthesis>

the result is unspecified.

2.6.1 Tilde Expansion

A "tilde-prefix" consists of an unquoted <tilde> character at the beginning of a word, followed by all of the characters preceding the first unquoted <slash> in the word, or all the characters in the word if there is no <slash>. In an assignment (see XBD V1_chap04(7)), multiple tilde-prefixes can be used: at the beginning of the word (that is, following the <equals-sign> of the assignment), following any unquoted <colon>, or both. A tilde-prefix in an assignment is terminated by the first unquoted <colon> or <slash>. If none of the characters in the tilde-prefix are quoted, the characters in the tilde-prefix following the <tilde> are treated as a possible login name from the user database. A portable login name cannot contain characters outside the set given in the description of the LOGNAME environment variable in XBD V1_chap08(7). If the login name is null (that is, the tilde-prefix contains only the tilde), the tilde-prefix is replaced by the value of the variable HOME. If HOME is unset, the results are unspecified. Otherwise, the tilde-prefix shall be replaced by a pathname of the initial working directory associated with the login name obtained using the getpwnam(3) function as defined in the System Interfaces volume of POSIX.1-2008. If the system does not recognize the login name, the results are undefined.

The pathname resulting from tilde expansion shall be treated as if quoted to prevent it being altered by field splitting and pathname expansion.

2.6.2 Parameter Expansion

The format for parameter expansion is as follows:

${expression}

where expression consists of all characters until the matching ‘}’. Any ‘}’ escaped by a <backslash> or within a quoted string, and characters in embedded arithmetic expansions, command substitutions, and variable expansions, shall not be examined in determining the matching ‘}’.

The simplest form for parameter expansion is:

${parameter}

The value, if any, of parameter shall be substituted.

The parameter name or symbol can be enclosed in braces, which are optional except for positional parameters with more than one digit or when parameter is a name and is followed by a character that could be interpreted as part of the name. The matching closing brace shall be determined by counting brace levels, skipping over enclosed quoted strings, and command substitutions.

If the parameter is not enclosed in braces, and is a name, the expansion shall use the longest valid name (see XBD V1_chap03(7)), whether or not the variable represented by that name exists. Otherwise, the parameter is a single-character symbol, and behavior is unspecified if that character is neither a digit nor one of the special parameters (see Special Parameters).

If a parameter expansion occurs inside double-quotes:

• Pathname expansion shall not be performed on the results of the expansion.
• Field splitting shall not be performed on the results of the expansion.

In addition, a parameter expansion can be modified by using one of the following formats. In each case that a value of word is needed (based on the state of parameter, as described below), word shall be subjected to tilde expansion, parameter expansion, command substitution, and arithmetic expansion. If word is not needed, it shall not be expanded. The ‘}’ character that delimits the following parameter expansion modifications shall be determined as described previously in this section and in Double-Quotes.

${parameter:-[word]}

Use Default Values . If parameter is unset or null, the expansion of word (or an empty string if word is omitted) shall be substituted; otherwise, the value of parameter shall be substituted.

${parameter:=[word]}

Assign Default Values . If parameter is unset or null, the expansion of word (or an empty string if word is omitted) shall be assigned to parameter . In all cases, the final value of parameter shall be substituted. Only variables, not positional parameters or special parameters, can be assigned in this way.

${parameter:?[word]}

Indicate Error if Null or Unset . If parameter is unset or null, the expansion of word (or a message indicating it is unset if word is omitted) shall be written to standard error and the shell exits with a non-zero exit status. Otherwise, the value of parameter shall be substituted. An interactive shell need not exit.

${parameter:+[word]}

Use Alternative Value . If parameter is unset or null, null shall be substituted; otherwise, the expansion of word (or an empty string if word is omitted) shall be substituted.

In the parameter expansions shown previously, use of the <colon> in the format shall result in a test for a parameter that is unset or null; omission of the <colon> shall result in a test for a parameter that is only unset. If parameter is ‘#’ and the colon is omitted, the application shall ensure that word is specified (this is necessary to avoid ambiguity with the string length expansion). The following table summarizes the effect of the <colon>:

In all cases shown with "substitute", the expression is replaced with the value shown. In all cases shown with "assign", parameter is assigned that value, which also replaces the expression.

${#parameter}

String Length . The length in characters of the value of parameter shall be substituted. If parameter is '*' or '@' , the result of the expansion is unspecified. If parameter is unset and set -u is in effect, the expansion shall fail.

The following four varieties of parameter expansion provide for substring processing. In each case, pattern matching notation (see Pattern Matching Notation), rather than regular expression notation, shall be used to evaluate the patterns. If parameter is ‘#’, ‘*’, or ‘@’, the result of the expansion is unspecified. If parameter is unset and set -u is in effect, the expansion shall fail. Enclosing the full parameter expansion string in double-quotes shall not cause the following four varieties of pattern characters to be quoted, whereas quoting characters within the braces shall have this effect. In each variety, if word is omitted, the empty pattern shall be used.

${parameter%[word]}

Remove Smallest Suffix Pattern . The word shall be expanded to produce a pattern. The parameter expansion shall then result in parameter , with the smallest portion of the suffix matched by the pattern deleted. If present, word shall not begin with an unquoted '%' .

${parameter%%[word]}

Remove Largest Suffix Pattern . The word shall be expanded to produce a pattern. The parameter expansion shall then result in parameter , with the largest portion of the suffix matched by the pattern deleted.

${parameter#[word]}

Remove Smallest Prefix Pattern . The word shall be expanded to produce a pattern. The parameter expansion shall then result in parameter , with the smallest portion of the prefix matched by the pattern deleted. If present, word shall not begin with an unquoted '#' .

${parameter##[word]}

Remove Largest Prefix Pattern . The word shall be expanded to produce a pattern. The parameter expansion shall then result in parameter , with the largest portion of the prefix matched by the pattern deleted.

The following sections are informative.

2.6.3 Command Substitution

Command substitution allows the output of a command to be substituted in place of the command name itself. Command substitution shall occur when the command is enclosed as follows:

$(command)

or (backquoted version):

`command`

The shell shall expand the command substitution by executing command in a subshell environment (see Shell Execution Environment) and replacing the command substitution (the text of command plus the enclosing "$()" or backquotes) with the standard output of the command, removing sequences of one or more <newline> characters at the end of the substitution. Embedded <newline> characters before the end of the output shall not be removed; however, they may be treated as field delimiters and eliminated during field splitting, depending on the value of IFS and quoting that is in effect. If the output contains any null bytes, the behavior is unspecified.

Within the backquoted style of command substitution, <backslash> shall retain its literal meaning, except when followed by: ‘$’, ‘`’, or <backslash>. The search for the matching backquote shall be satisfied by the first unquoted non-escaped backquote; during this search, if a non-escaped backquote is encountered within a shell comment, a here-document, an embedded command substitution of the $( command) form, or a quoted string, undefined results occur. A single-quoted or double-quoted string that begins, but does not end, within the "`...`" sequence produces undefined results.

With the $( command) form, all characters following the open parenthesis to the matching closing parenthesis constitute the command. Any valid shell script can be used for command, except a script consisting solely of redirections which produces unspecified results.

The results of command substitution shall not be processed for further tilde expansion, parameter expansion, command substitution, or arithmetic expansion. If a command substitution occurs inside double-quotes, field splitting and pathname expansion shall not be performed on the results of the substitution.

Command substitution can be nested. To specify nesting within the backquoted version, the application shall precede the inner backquotes with <backslash> characters; for example:

\`command\`

The syntax of the shell command language has an ambiguity for expansions beginning with "$((", which can introduce an arithmetic expansion or a command substitution that starts with a subshell. Arithmetic expansion has precedence; that is, the shell shall first determine whether it can parse the expansion as an arithmetic expansion and shall only parse the expansion as a command substitution if it determines that it cannot parse the expansion as an arithmetic expansion. The shell need not evaluate nested expansions when performing this determination. If it encounters the end of input without already having determined that it cannot parse the expansion as an arithmetic expansion, the shell shall treat the expansion as an incomplete arithmetic expansion and report a syntax error. A conforming application shall ensure that it separates the "$(" and (‘’ into two tokens (that is, separate them with white space) in a command substitution that starts with a subshell. For example, a command substitution containing a single subshell could be written as:

$( (command) )

2.6.4 Arithmetic Expansion

Arithmetic expansion provides a mechanism for evaluating an arithmetic expression and substituting its value. The format for arithmetic expansion shall be as follows:

$((expression))

The expression shall be treated as if it were in double-quotes, except that a double-quote inside the expression is not treated specially. The shell shall expand all tokens in the expression for parameter expansion, command substitution, and quote removal.

Next, the shell shall treat this as an arithmetic expression and substitute the value of the expression. The arithmetic expression shall be processed according to the rules given in Arithmetic Precision and Operations, with the following exceptions:

• Only signed long integer arithmetic is required.
• Only the decimal-constant, octal-constant, and hexadecimal-constant constants specified in the ISO C standard, Section 6.4.4.1 are required to be recognized as constants.
• The sizeof () operator and the prefix and postfix "++" and "--" operators are not required.
• Selection, iteration, and jump statements are not supported.

All changes to variables in an arithmetic expression shall be in effect after the arithmetic expansion, as in the parameter expansion "${x=value}".

If the shell variable x contains a value that forms a valid integer constant, optionally including a leading <plus-sign> or <hyphen-minus>, then the arithmetic expansions "$((x))" and "$(($x))" shall return the same value.

As an extension, the shell may recognize arithmetic expressions beyond those listed. The shell may use a signed integer type with a rank larger than the rank of signed long. The shell may use a real-floating type instead of signed long as long as it does not affect the results in cases where there is no overflow. If the expression is invalid, or the contents of a shell variable used in the expression are not recognized by the shell, the expansion fails and the shell shall write a diagnostic message to standard error indicating the failure.

The following sections are informative.

# repeat a command 100 times
x=100
while [ $x -gt 0 ]
do
    command    x=$(($x-1))
done

2.6.5 Field Splitting

After parameter expansion ( Parameter Expansion), command substitution ( Command Substitution), and arithmetic expansion ( Arithmetic Expansion), the shell shall scan the results of expansions and substitutions that did not occur in double-quotes for field splitting and multiple fields can result.

The shell shall treat each character of the IFS as a delimiter and use the delimiters as field terminators to split the results of parameter expansion, command substitution, and arithmetic expansion into fields.

1. If the value of IFS is a <space>, <tab>, and <newline>, or if it is unset, any sequence of <space>, <tab>, or <newline> characters at the beginning or end of the input shall be ignored and any sequence of those characters within the input shall delimit a field. For example, the input: foobar

   yields two fields, foo and bar.

2. If the value of IFS is null, no field splitting shall be performed.
3. Otherwise, the following rules shall be applied in sequence. The term " IFS white space" is used to mean any sequence (zero or more instances) of white-space characters that are in the IFS value (for example, if IFS contains <space>/ <comma>/ <tab>, any sequence of <space> and <tab> characters is considered IFS white space).
   1. IFS white space shall be ignored at the beginning and end of the input.
   2. Each occurrence in the input of an IFS character that is not IFS white space, along with any adjacent IFS white space, shall delimit a field, as described previously.
   3. Non-zero-length IFS white space shall delimit a field.

2.6.6 Pathname Expansion

After field splitting, if set -f is not in effect, each field in the resulting command line shall be expanded using the algorithm described in Pattern Matching Notation, qualified by the rules in Patterns Used for Filename Expansion.

2.6.7 Quote Removal

The quote characters ( <backslash>, single-quote, and double-quote) that were present in the original word shall be removed unless they have themselves been quoted.

2.7 Redirection

Redirection is used to open and close files for the current shell execution environment (see Shell Execution Environment) or for any command. Redirection operators can be used with numbers representing file descriptors (see XBD V1_chap03(7)) as described below.

The overall format used for redirection is:

[n]redir-op word

The number n is an optional decimal number designating the file descriptor number; the application shall ensure it is delimited from any preceding text and immediately precede the redirection operator redir-op. If n is quoted, the number shall not be recognized as part of the redirection expression. For example:

echo \2>a

writes the character 2 into file a. If any part of redir-op is quoted, no redirection expression is recognized. For example:

echo 2\>a

writes the characters 2> a to standard output. The optional number, redirection operator, and word shall not appear in the arguments provided to the command to be executed (if any).

Open files are represented by decimal numbers starting with zero. The largest possible value is implementation-defined; however, all implementations shall support at least 0 to 9, inclusive, for use by the application. These numbers are called "file descriptors". The values 0, 1, and 2 have special meaning and conventional uses and are implied by certain redirection operations; they are referred to as standard input, standard output, and standard error, respectively. Programs usually take their input from standard input, and write output on standard output. Error messages are usually written on standard error. The redirection operators can be preceded by one or more digits (with no intervening <blank> characters allowed) to designate the file descriptor number.

If the redirection operator is "<<" or "<<-", the word that follows the redirection operator shall be subjected to quote removal; it is unspecified whether any of the other expansions occur. For the other redirection operators, the word that follows the redirection operator shall be subjected to tilde expansion, parameter expansion, command substitution, arithmetic expansion, and quote removal. Pathname expansion shall not be performed on the word by a non-interactive shell; an interactive shell may perform it, but shall do so only when the expansion would result in one word.

If more than one redirection operator is specified with a command, the order of evaluation is from beginning to end.

A failure to open or create a file shall cause a redirection to fail.

2.7.1 Redirecting Input

Input redirection shall cause the file whose name results from the expansion of word to be opened for reading on the designated file descriptor, or standard input if the file descriptor is not specified.

The general format for redirecting input is:

[n]<word

where the optional n represents the file descriptor number. If the number is omitted, the redirection shall refer to standard input (file descriptor 0).

2.7.2 Redirecting Output

The two general formats for redirecting output are:

[n]>word
[n]>|word

where the optional n represents the file descriptor number. If the number is omitted, the redirection shall refer to standard output (file descriptor 1).

Output redirection using the ‘>’ format shall fail if the noclobber option is set (see the description of set -C) and the file named by the expansion of word exists and is a regular file. Otherwise, redirection using the ‘>’ or ">|" formats shall cause the file whose name results from the expansion of word to be created and opened for output on the designated file descriptor, or standard output if none is specified. If the file does not exist, it shall be created; otherwise, it shall be truncated to be an empty file after being opened.

2.7.3 Appending Redirected Output

Appended output redirection shall cause the file whose name results from the expansion of word to be opened for output on the designated file descriptor. The file is opened as if the open(3) function as defined in the System Interfaces volume of POSIX.1-2008 was called with the O_APPEND flag. If the file does not exist, it shall be created.

The general format for appending redirected output is as follows:

[n]>>word

where the optional n represents the file descriptor number. If the number is omitted, the redirection refers to standard output (file descriptor 1).

2.7.4 Here-Document

The redirection operators "<<" and "<<-" both allow redirection of subsequent lines read by the shell to the input of a command. The redirected lines are known as a "here-document".

The here-document shall be treated as a single word that begins after the next <newline> and continues until there is a line containing only the delimiter and a <newline>, with no <blank> characters in between. Then the next here-document starts, if there is one. The format is as follows:

[n]<<word
    here-document
delimiter

where the optional n represents the file descriptor number. If the number is omitted, the here-document refers to standard input (file descriptor 0). It is unspecified whether the file descriptor is opened as a regular file, a special file, or a pipe. Portable applications cannot rely on the file descriptor being seekable (see XSH lseek(3)).

If any part of word is quoted, the delimiter shall be formed by performing quote removal on word, and the here-document lines shall not be expanded. Otherwise, the delimiter shall be the word itself.

If no part of word is quoted, all lines of the here-document shall be expanded for parameter expansion, command substitution, and arithmetic expansion. In this case, the <backslash> in the input behaves as the <backslash> inside double-quotes (see Double-Quotes). However, the double-quote character ( ‘’) shall not be treated specially within a here-document, except when the double-quote appears within "$()", "``", or "${}".

If the redirection operator is "<<-", all leading <tab> characters shall be stripped from input lines and the line containing the trailing delimiter. If more than one "<<" or "<<-" operator is specified on a line, the here-document associated with the first operator shall be supplied first by the application and shall be read first by the shell.

When a here-document is read from a terminal device and the shell is interactive, it shall write the contents of the variable PS2, processed as described in Shell Variables, to standard error before reading each line of input until the delimiter has been recognized.

The following sections are informative.

cat <<eof1; cat <<eof2
Hi,
eof1
Helene.
eof2

2.7.5 Duplicating an Input File Descriptor

The redirection operator:

[n]<&word

shall duplicate one input file descriptor from another, or shall close one. If word evaluates to one or more digits, the file descriptor denoted by n, or standard input if n is not specified, shall be made to be a copy of the file descriptor denoted by word; if the digits in word do not represent a file descriptor already open for input, a redirection error shall result; see Consequences of Shell Errors. If word evaluates to ‘-’, file descriptor n, or standard input if n is not specified, shall be closed. Attempts to close a file descriptor that is not open shall not constitute an error. If word evaluates to something else, the behavior is unspecified.

2.7.6 Duplicating an Output File Descriptor

The redirection operator:

[n]>&word

shall duplicate one output file descriptor from another, or shall close one. If word evaluates to one or more digits, the file descriptor denoted by n, or standard output if n is not specified, shall be made to be a copy of the file descriptor denoted by word; if the digits in word do not represent a file descriptor already open for output, a redirection error shall result; see Consequences of Shell Errors. If word evaluates to ‘-’, file descriptor n, or standard output if n is not specified, is closed. Attempts to close a file descriptor that is not open shall not constitute an error. If word evaluates to something else, the behavior is unspecified.

2.7.7 Open File Descriptors for Reading and Writing

The redirection operator:

[n]<>word

shall cause the file whose name is the expansion of word to be opened for both reading and writing on the file descriptor denoted by n, or standard input if n is not specified. If the file does not exist, it shall be created.

2.8 Exit Status and Errors

2.8.1 Consequences of Shell Errors

Certain errors shall cause the shell to write a diagnostic message to standard error and exit as shown in the following table:

1. Although special built-ins are part of the shell, a diagnostic message written by a special built-in is not considered to be a shell diagnostic message, and can be redirected like any other utility.
2. The shell is not required to write a diagnostic message, but the utility itself shall write a diagnostic message if required to do so.
3. A future version of this standard may require the shell to not exit in this condition.

An expansion error is one that occurs when the shell expansions define in wordexp(3) are carried out (for example, "${x!y}", because ‘’! is not a valid operator); an implementation may treat these as syntax errors if it is able to detect them during tokenization, rather than during expansion.

If any of the errors shown as "shall exit" or "may exit" occur in a subshell environment, the shell shall (respectively, may) exit from the subshell environment with a non-zero status and continue in the environment from which that subshell environment was invoked.

In all of the cases shown in the table where an interactive shell is required not to exit, the shell shall not perform any further processing of the command in which the error occurred.

2.8.2 Exit Status for Commands

Each command has an exit status that can influence the behavior of other shell commands. The exit status of commands that are not utilities is documented in this section. The exit status of the standard utilities is documented in their respective sections.

If a command is not found, the exit status shall be 127. If the command name is found, but it is not an executable utility, the exit status shall be 126. Applications that invoke utilities without using the shell should use these exit status values to report similar errors.

If a command fails during word expansion or redirection, its exit status shall be between 1 and 125 inclusive.

Internally, for purposes of deciding whether a command exits with a non-zero exit status, the shell shall recognize the entire status value retrieved for the command by the equivalent of the wait(3) function WEXITSTATUS macro (as defined in the System Interfaces volume of POSIX.1-2008). When reporting the exit status with the special parameter ‘’?, the shell shall report the full eight bits of exit status available. The exit status of a command that terminated because it received a signal shall be reported as greater than 128.

2.9 Shell Commands

This section describes the basic structure of shell commands. The following command descriptions each describe a format of the command that is only used to aid the reader in recognizing the command type, and does not formally represent the syntax. In particular, the representations include spacing between tokens in some places where <blank>s would not be necessary (when one of the tokens is an operator). Each description discusses the semantics of the command; for a formal definition of the command language, consult Shell Grammar.

A command is one of the following:

• Simple command (see Simple Commands)
• Pipeline (see Pipelines)
• List compound-list (see Lists)
• Compound command (see Compound Commands)
• Function definition (see Function Definition Command)

Unless otherwise stated, the exit status of a command shall be that of the last simple command executed by the command. There shall be no limit on the size of any shell command other than that imposed by the underlying system (memory constraints, {ARG_MAX}, and so on).

2.9.1 Simple Commands

A "simple command" is a sequence of optional variable assignments and redirections, in any sequence, optionally followed by words and redirections, terminated by a control operator.

When a given simple command is required to be executed (that is, when any conditional construct such as an AND-OR list or a case statement has not bypassed the simple command), the following expansions, assignments, and redirections shall all be performed from the beginning of the command text to the end:

1. The words that are recognized as variable assignments or redirections according to Shell Grammar Rules are saved for processing in steps 3 and 4.
2. The words that are not variable assignments or redirections shall be expanded. If any fields remain following their expansion, the first field shall be considered the command name and remaining fields are the arguments for the command.
3. Redirections shall be performed as described in Redirection.
4. Each variable assignment shall be expanded for tilde expansion, parameter expansion, command substitution, arithmetic expansion, and quote removal prior to assigning the value.

In the preceding list, the order of steps 3 and 4 may be reversed if no command name results from step 2 or if the command name matches the name of a special built-in utility; see Special Built-In Utilities.

Variable assignments shall be performed as follows:

• If no command name results, variable assignments shall affect the current execution environment.
• If the command name is not a special built-in utility or function, the variable assignments shall be exported for the execution environment of the command and shall not affect the current execution environment except as a side-effect of the expansions performed in step 4. In this case it is unspecified:
  • Whether or not the assignments are visible for subsequent expansions in step 4
  • Whether variable assignments made as side-effects of these expansions are visible for subsequent expansions in step 4, or in the current shell execution environment, or both
• If the command name is a standard utility implemented as a function (see XBD V1_chap04(7)), the effect of variable assignments shall be as if the utility was not implemented as a function.
• If the command name is a special built-in utility, variable assignments shall affect the current execution environment. Unless the set -a option is on (see set), it is unspecified:
  • Whether or not the variables gain the export attribute during the execution of the special built-in utility
  • Whether or not export attributes gained as a result of the variable assignments persist after the completion of the special built-in utility
• If the command name is a function that is not a standard utility implemented as a function, variable assignments shall affect the current execution environment during the execution of the function. It is unspecified:
  • Whether or not the variable assignments persist after the completion of the function
  • Whether or not the variables gain the export attribute during the execution of the function
  • Whether or not export attributes gained as a result of the variable assignments persist after the completion of the function (if variable assignments persist after the completion of the function)

If any of the variable assignments attempt to assign a value to a variable for which the readonly attribute is set in the current shell environment (regardless of whether the assignment is made in that environment), a variable assignment error shall occur. See Consequences of Shell Errors for the consequences of these errors.

If there is no command name, any redirections shall be performed in a subshell environment; it is unspecified whether this subshell environment is the same one as that used for a command substitution within the command. (To affect the current execution environment, see the exec special built-in.) If any of the redirections performed in the current shell execution environment fail, the command shall immediately fail with an exit status greater than zero, and the shell shall write an error message indicating the failure. See Consequences of Shell Errors for the consequences of these failures on interactive and non-interactive shells.

If there is a command name, execution shall continue as described in Command Search and Execution . If there is no command name, but the command contained a command substitution, the command shall complete with the exit status of the last command substitution performed. Otherwise, the command shall complete with a zero exit status.

2.9.2 Pipelines

A pipeline is a sequence of one or more commands separated by the control operator ‘|’. For each command but the last, the shell shall connect the standard output of the command to the standard input of the next command as if by creating a pipe and passing the write end of the pipe as the standard output of the command and the read end of the pipe as the standard input of the next command.

The format for a pipeline is:

[!] command1 [ | command2 ...]

If the pipeline begins with the reserved word ! and command1 is a subshell command, the application shall ensure that the ( operator at the beginning of command1 is separated from the ! by one or more <blank> characters. The behavior of the reserved word ! immediately followed by the ( operator is unspecified.

The standard output of command1 shall be connected to the standard input of command2. The standard input, standard output, or both of a command shall be considered to be assigned by the pipeline before any redirection specified by redirection operators that are part of the command (see Redirection).

If the pipeline is not in the background (see Asynchronous Lists), the shell shall wait for the last command specified in the pipeline to complete, and may also wait for all commands to complete.

2.9.3 Lists

An AND-OR list is a sequence of one or more pipelines separated by the operators "&&" and "||" .

A list is a sequence of one or more AND-OR lists separated by the operators ‘’; and ‘&’.

The operators "&&" and "||" shall have equal precedence and shall be evaluated with left associativity. For example, both of the following commands write solely bar to standard output:

false && echo foo || echo bar
true || echo foo && echo bar

A ‘’; separator or a ‘’; or <newline> terminator shall cause the preceding AND-OR list to be executed sequentially; an ‘&’ separator or terminator shall cause asynchronous execution of the preceding AND-OR list.

The term "compound-list" is derived from the grammar in Shell Grammar; it is equivalent to a sequence of lists, separated by <newline> characters, that can be preceded or followed by an arbitrary number of <newline> characters.

The following sections are informative.

while
    # a couple of <newline>s


    # a list
    date && who || ls; cat file
    # a couple of <newline>s


    # another list
    wc file > output & true


do
    # 2 lists
    ls
    cat file
done

command1 & [command2 & ... ]

command1 [; command2] ...

command1 [ && command2] ...

command1 [ || command2] ...

2.9.4 Compound Commands

The shell has several programming constructs that are "compound commands", which provide control flow for commands. Each of these compound commands has a reserved word or control operator at the beginning, and a corresponding terminator reserved word or operator at the end. In addition, each can be followed by redirections on the same line as the terminator. Each redirection shall apply to all the commands within the compound command that do not explicitly override that redirection.

for name [ in [word ... ]]
do
    compound-list
done

in word...

in "$@"

case word in
    [(] pattern1 ) compound-list ;;
    [[(] pattern[ | pattern] ... ) compound-list ;;] ...
    [[(] pattern[ | pattern] ... ) compound-list]
esac

if compound-list
then
    compound-list
[elif compound-list
then
    compound-list] ...
[else
    compound-list]
fi

while compound-list-1
do
    compound-list-2
done

until compound-list-1
do
    compound-list-2
done

2.9.5 Function Definition Command

A function is a user-defined name that is used as a simple command to call a compound command with new positional parameters. A function is defined with a "function definition command".

The format of a function definition command is as follows:

fname ( ) compound-command [io-redirect ...]

The function is named fname; the application shall ensure that it is a name (see XBD V1_chap03(7)) and that it is not the name of a special built-in utility. An implementation may allow other characters in a function name as an extension. The implementation shall maintain separate name spaces for functions and variables.

The argument compound-command represents a compound command, as described in Compound Commands.

When the function is declared, none of the expansions in wordexp(3) shall be performed on the text in compound-command or io-redirect; all expansions shall be performed as normal each time the function is called. Similarly, the optional io-redirect redirections and any variable assignments within compound-command shall be performed during the execution of the function itself, not the function definition. See Consequences of Shell Errors for the consequences of failures of these operations on interactive and non-interactive shells.

When a function is executed, it shall have the syntax-error properties described for special built-in utilities in the first item in the enumerated list at the beginning of Special Built-In Utilities.

The compound-command shall be executed whenever the function name is specified as the name of a simple command (see Command Search and Execution). The operands to the command temporarily shall become the positional parameters during the execution of the compound-command; the special parameter ‘#’ also shall be changed to reflect the number of operands. The special parameter 0 shall be unchanged. When the function completes, the values of the positional parameters and the special parameter ‘#’ shall be restored to the values they had before the function was executed. If the special built-in return (see return) is executed in the compound-command, the function completes and execution shall resume with the next command after the function call.

2.10. Shell Grammar

The following grammar defines the Shell Command Language. This formal syntax shall take precedence over the preceding text syntax description.

2.10.1 Shell Grammar Lexical Conventions

The input language to the shell must be first recognized at the character level. The resulting tokens shall be classified by their immediate context according to the following rules (applied in order). These rules shall be used to determine what a "token" is that is subject to parsing at the token level. The rules for token recognition in Token Recognition shall apply.

1. If the token is an operator, the token identifier for that operator shall result.
2. If the string consists solely of digits and the delimiter character is one of ‘<’ or ‘>’, the token identifier IO_NUMBER shall be returned.
3. Otherwise, the token identifier TOKEN results.

Further distinction on TOKEN is context-dependent. It may be that the same TOKEN yields WORD, a NAME, an ASSIGNMENT_WORD, or one of the reserved words below, dependent upon the context. Some of the productions in the grammar below are annotated with a rule number from the following list. When a TOKEN is seen where one of those annotated productions could be used to reduce the symbol, the applicable rule shall be applied to convert the token identifier type of the TOKEN to a token identifier acceptable at that point in the grammar. The reduction shall then proceed based upon the token identifier type yielded by the rule applied. When more than one rule applies, the highest numbered rule shall apply (which in turn may refer to another rule). (Note that except in rule 7, the presence of an ‘=’ in the token has no effect.)

The WORD tokens shall have the word expansion rules applied to them immediately before the associated command is executed, not at the time the command is parsed.

2.10.2 Shell Grammar Rules

1. [Command Name]

   When the TOKEN is exactly a reserved word, the token identifier for that reserved word shall result. Otherwise, the token WORD shall be returned. Also, if the parser is in any state where only a reserved word could be the next correct token, proceed as above.

   Note:

   Because at this point <quotation-mark> characters are retained in the token, quoted strings cannot be recognized as reserved words. This rule also implies that reserved words are not recognized except in certain positions in the input, such as after a <newline> or <semicolon>; the grammar presumes that if the reserved word is intended, it is properly delimited by the user, and does not attempt to reflect that requirement directly. Also note that line joining is done before tokenization, as described in Escape Character (Backslash) , so escaped <newline> characters are already removed at this point.

   Rule 1 is not directly referenced in the grammar, but is referred to by other rules, or applies globally.

2. [Redirection to or from filename]

   The expansions specified in Redirection shall occur. As specified there, exactly one field can result (or the result is unspecified), and there are additional requirements on pathname expansion.

3. [Redirection from here-document]

   Quote removal shall be applied to the word to determine the delimiter that is used to find the end of the here-document that begins after the next <newline>.

4. [Case statement termination]

   When the TOKEN is exactly the reserved word esac, the token identifier for esac shall result. Otherwise, the token WORD shall be returned.

5. [ NAME in for]

   When the TOKEN meets the requirements for a name (see XBD V1_chap03(7) ), the token identifier NAME shall result. Otherwise, the token WORD shall be returned.

6. [Third word of for and case]
   1. [ case only]

      When the TOKEN is exactly the reserved word in, the token identifier for in shall result. Otherwise, the token WORD shall be returned.

   2. [ for only]

      When the TOKEN is exactly the reserved word in or do, the token identifier for in or do shall result, respectively. Otherwise, the token WORD shall be returned.

   (For a. and b.: As indicated in the grammar, a linebreak precedes the tokens in and do. If <newline> characters are present at the indicated location, it is the token after them that is treated in this fashion.)

7. [Assignment preceding command name]
   1. [When the first word]

      If the TOKEN does not contain the character ‘=’, rule 1 is applied. Otherwise, 7b shall be applied.

   2. [Not the first word]

      If the TOKEN contains an unquoted (as determined while applying rule 4 from Token Recognition) <equals-sign> character that is not part of an embedded parameter expansion, command substitution, or arithmetic expansion construct (as determined while applying rule 5 from Token Recognition):

      • If the TOKEN begins with ‘=’, then rule 1 shall be applied.
      • If all the characters in the TOKEN preceding the first such <equals-sign> form a valid name (see XBD V1_chap03(7)), the token ASSIGNMENT_WORD shall be returned.
      • Otherwise, it is unspecified whether rule 1 is applied or ASSIGNMENT_WORD is returned.

      Otherwise, rule 1 shall be applied.

   Assignment to the name within a returned ASSIGNMENT_WORD token shall occur as specified in Simple Commands.

8. [ NAME in function]

   When the TOKEN is exactly a reserved word, the token identifier for that reserved word shall result. Otherwise, when the TOKEN meets the requirements for a name, the token identifier NAME shall result. Otherwise, rule 7 applies.

9. [Body of function]

   Word expansion and assignment shall never occur, even when required by the rules above, when this rule is being parsed. Each TOKEN that might either be expanded or have assignment applied to it shall instead be returned as a single WORD consisting only of characters that are exactly the token described in Token Recognition.

/* -------------------------------------------------------
   The grammar symbols
   ------------------------------------------------------- */
%token  WORD
%token  ASSIGNMENT_WORD
%token  NAME
%token  NEWLINE
%token  IO_NUMBER


/* The following are the operators (see XBD Operator)
   containing more than one character. */

%token  AND_IF    OR_IF    DSEMI
/*      '&&'      '||'     ';;'    */


%token  DLESS  DGREAT  LESSAND  GREATAND  LESSGREAT  DLESSDASH
/*      '<<'   '>>'    '<&'     '>&'      '<>'       '<<-'   */


%token  CLOBBER
/*      '>|'   */


/* The following are the reserved words. */


%token  If    Then    Else    Elif    Fi    Do    Done
/*      'if'  'then'  'else'  'elif'  'fi'  'do'  'done'   */


%token  Case    Esac    While    Until    For
/*      'case'  'esac'  'while'  'until'  'for'   */


/* These are reserved words, not operator tokens, and are
   recognized when reserved words are recognized. */


%token  Lbrace    Rbrace    Bang
/*      '{'       '}'       '!'   */


%token  In
/*      'in'   */


/* -------------------------------------------------------
   The Grammar
   ------------------------------------------------------- */
%start program
%%
program          : linebreak complete_commands linebreak
                 | linebreak
                 ;
complete_commands: complete_commands newline_list complete_command
                 |                                complete_command
                 ;
complete_command : list separator_op
                 | list
                 ;
list             : list separator_op and_or
                 |                   and_or
                 ;
and_or           :                         pipeline
                 | and_or AND_IF linebreak pipeline
                 | and_or OR_IF  linebreak pipeline
                 ;
pipeline         :      pipe_sequence
                 | Bang pipe_sequence
                 ;
pipe_sequence    :                             command
                 | pipe_sequence '|' linebreak command
                 ;
command          : simple_command
                 | compound_command
                 | compound_command redirect_list
                 | function_definition
                 ;
compound_command : brace_group
                 | subshell
                 | for_clause
                 | case_clause
                 | if_clause
                 | while_clause
                 | until_clause
                 ;
subshell         : '(' compound_list ')'
                 ;
compound_list    : linebreak term
                 | linebreak term separator
                 ;
term             : term separator and_or
                 |                and_or
                 ;
for_clause       : For name                                      do_group
                 | For name                       sequential_sep do_group
                 | For name linebreak in          sequential_sep do_group
                 | For name linebreak in wordlist sequential_sep do_group
                 ;
name             : NAME                     /* Apply rule 5 */
                 ;
in               : In                       /* Apply rule 6 */
                 ;
wordlist         : wordlist WORD
                 |          WORD
                 ;
case_clause      : Case WORD linebreak in linebreak case_list    Esac
                 | Case WORD linebreak in linebreak case_list_ns Esac
                 | Case WORD linebreak in linebreak              Esac
                 ;
case_list_ns     : case_list case_item_ns
                 |           case_item_ns
                 ;
case_list        : case_list case_item
                 |           case_item
                 ;
case_item_ns     :     pattern ')' linebreak
                 |     pattern ')' compound_list
                 | '(' pattern ')' linebreak
                 | '(' pattern ')' compound_list
                 ;
case_item        :     pattern ')' linebreak     DSEMI linebreak
                 |     pattern ')' compound_list DSEMI linebreak
                 | '(' pattern ')' linebreak     DSEMI linebreak
                 | '(' pattern ')' compound_list DSEMI linebreak
                 ;
pattern          :             WORD         /* Apply rule 4 */
                 | pattern '|' WORD         /* Do not apply rule 4 */
                 ;
if_clause        : If compound_list Then compound_list else_part Fi
                 | If compound_list Then compound_list           Fi
                 ;
else_part        : Elif compound_list Then compound_list
                 | Elif compound_list Then compound_list else_part
                 | Else compound_list
                 ;
while_clause     : While compound_list do_group
                 ;
until_clause     : Until compound_list do_group
                 ;
function_definition : fname '(' ')' linebreak function_body
                 ;
function_body    : compound_command                /* Apply rule 9 */
                 | compound_command redirect_list  /* Apply rule 9 */
                 ;
fname            : NAME                            /* Apply rule 8 */
                 ;
brace_group      : Lbrace compound_list Rbrace
                 ;
do_group         : Do compound_list Done           /* Apply rule 6 */
                 ;
simple_command   : cmd_prefix cmd_word cmd_suffix
                 | cmd_prefix cmd_word
                 | cmd_prefix
                 | cmd_name cmd_suffix
                 | cmd_name
                 ;
cmd_name         : WORD                   /* Apply rule 7a */
                 ;
cmd_word         : WORD                   /* Apply rule 7b */
                 ;
cmd_prefix       :            io_redirect
                 | cmd_prefix io_redirect
                 |            ASSIGNMENT_WORD
                 | cmd_prefix ASSIGNMENT_WORD
                 ;
cmd_suffix       :            io_redirect
                 | cmd_suffix io_redirect
                 |            WORD
                 | cmd_suffix WORD
                 ;
redirect_list    :               io_redirect
                 | redirect_list io_redirect
                 ;
io_redirect      :           io_file
                 | IO_NUMBER io_file
                 |           io_here
                 | IO_NUMBER io_here
                 ;
io_file          : '<'       filename
                 | LESSAND   filename
                 | '>'       filename
                 | GREATAND  filename
                 | DGREAT    filename
                 | LESSGREAT filename
                 | CLOBBER   filename
                 ;
filename         : WORD                      /* Apply rule 2 */
                 ;
io_here          : DLESS     here_end
                 | DLESSDASH here_end
                 ;
here_end         : WORD                      /* Apply rule 3 */
                 ;
newline_list     :              NEWLINE
                 | newline_list NEWLINE
                 ;
linebreak        : newline_list
                 | /* empty */
                 ;
separator_op     : '&'
                 | ';'
                 ;
separator        : separator_op linebreak
                 | newline_list
                 ;
sequential_sep   : ';' linebreak
                 | newline_list
                 ;

2.11. Signals and Error Handling

If job control is disabled (see the description of set -m) when the shell executes an asynchronous list, the commands in the list shall inherit from the shell a signal action of ignored (SIG_IGN) for the SIGINT and SIGQUIT signals. In all other cases, commands executed by the shell shall inherit the same signal actions as those inherited by the shell from its parent unless a signal action is modified by the trap special built-in (see trap)

When a signal for which a trap has been set is received while the shell is waiting for the completion of a utility executing a foreground command, the trap associated with that signal shall not be executed until after the foreground command has completed. When the shell is waiting, by means of the wait(1) utility, for asynchronous commands to complete, the reception of a signal for which a trap has been set shall cause the wait(1) utility to return immediately with an exit status >128, immediately after which the trap associated with that signal shall be taken.

If multiple signals are pending for the shell for which there are associated trap actions, the order of execution of trap actions is unspecified.

2.12. Shell Execution Environment

A shell execution environment consists of the following:

• Open files inherited upon invocation of the shell, plus open files controlled by exec
• Working directory as set by cd(1)
• File creation mask set by umask(1)
• [XSI] File size limit as set by ulimit(1)
• Current traps set by trap
• Shell parameters that are set by variable assignment (see the set special built-in) or from the System Interfaces volume of POSIX.1-2008 environment inherited by the shell when it begins (see the export special built-in)
• Shell functions; see Function Definition Command
• Options turned on at invocation or by set
• Process IDs of the last commands in asynchronous lists known to this shell environment; see Asynchronous Lists
• Shell aliases; see Alias Substitution

Utilities other than the special built-ins (see Special Built-In Utilities) shall be invoked in a separate environment that consists of the following. The initial value of these objects shall be the same as that for the parent shell, except as noted below.

• Open files inherited on invocation of the shell, open files controlled by the exec special built-in plus any modifications, and additions specified by any redirections to the utility
• Current working directory
• File creation mask
• If the utility is a shell script, traps caught by the shell shall be set to the default values and traps ignored by the shell shall be set to be ignored by the utility; if the utility is not a shell script, the trap actions (default or ignore) shall be mapped into the appropriate signal handling actions for the utility
• Variables with the export attribute, along with those explicitly exported for the duration of the command, shall be passed to the utility environment variables

The environment of the shell process shall not be changed by the utility unless explicitly specified by the utility description (for example, cd(1) and umask(1)).

A subshell environment shall be created as a duplicate of the shell environment, except that signal traps that are not being ignored shall be set to the default action. Changes made to the subshell environment shall not affect the shell environment. Command substitution, commands that are grouped with parentheses, and asynchronous lists shall be executed in a subshell environment. Additionally, each command of a multi-command pipeline is in a subshell environment; as an extension, however, any or all commands in a pipeline may be executed in the current environment. All other commands shall be executed in the current shell environment.

2.13. Pattern Matching Notation

The pattern matching notation described in this section is used to specify patterns for matching strings in the shell. Historically, pattern matching notation is related to, but slightly different from, the regular expression notation described in XBD V1_chap09(7). For this reason, the description of the rules for this pattern matching notation are based on the description of regular expression notation, modified to account for the differences.

2.13.1 Patterns Matching a Single Character

The following patterns matching a single character shall match a single character: ordinary characters, special pattern characters, and pattern bracket expressions. The pattern bracket expression also shall match a single collating element. A <backslash> character shall escape the following character. The escaping <backslash> shall be discarded. If a pattern ends with an unescaped <backslash>, it is unspecified whether the pattern does not match anything or the pattern is treated as invalid.

An ordinary character is a pattern that shall match itself. It can be any character in the supported character set except for NUL, those special shell characters in Quoting that require quoting, and the following three special pattern characters. Matching shall be based on the bit pattern used for encoding the character, not on the graphic representation of the character. If any character (ordinary, shell special, or pattern special) is quoted, that pattern shall match the character itself. The shell special characters always require quoting.

When unquoted and outside a bracket expression, the following three characters shall have special meaning in the specification of patterns:

?

A <question-mark> is a pattern that shall match any character.

*

An <asterisk> is a pattern that shall match multiple characters, as described in Patterns Matching Multiple Characters .

[

If an open bracket introduces a bracket expression as in XBD RE Bracket Expression , except that the <exclamation-mark> character ( '!' ) shall replace the <circumflex> character ( '^' ) in its role in a non-matching list in the regular expression notation, it shall introduce a pattern bracket expression. A bracket expression starting with an unquoted <circumflex> character produces unspecified results. Otherwise, '[' shall match the character itself.

When pattern matching is used where shell quote removal is not performed (such as in the argument to the find(1) - name primary when find(1) is being called using one of the exec functions as defined in the System Interfaces volume of POSIX.1-2008, or in the pattern argument to the fnmatch(3) function), special characters can be escaped to remove their special meaning by preceding them with a <backslash> character. This escaping <backslash> is discarded. The sequence "\\" represents one literal <backslash>. All of the requirements and effects of quoting on ordinary, shell special, and special pattern characters shall apply to escaping in this context.

2.13.2 Patterns Matching Multiple Characters

The following rules are used to construct patterns matching multiple characters from patterns matching a single character:

1. The <asterisk> ( ‘*’ ) is a pattern that shall match any string, including the null string.
2. The concatenation of patterns matching a single character is a valid pattern that shall match the concatenation of the single characters or collating elements matched by each of the concatenated patterns.
3. The concatenation of one or more patterns matching a single character with one or more <asterisk> characters is a valid pattern. In such patterns, each <asterisk> shall match a string of zero or more characters, matching the greatest possible number of characters that still allows the remainder of the pattern to match the string.

2.13.3 Patterns Used for Filename Expansion

The rules described so far in Patterns Matching a Single Character and Patterns Matching Multiple Characters are qualified by the following rules that apply when pattern matching notation is used for filename expansion:

1. The <slash> character in a pathname shall be explicitly matched by using one or more <slash> characters in the pattern; it shall neither be matched by the <asterisk> or <question-mark> special characters nor by a bracket expression. <slash> characters in the pattern shall be identified before bracket expressions; thus, a <slash> cannot be included in a pattern bracket expression used for filename expansion. If a <slash> character is found following an unescaped <left-square-bracket> character before a corresponding <right-square-bracket> is found, the open bracket shall be treated as an ordinary character. For example, the pattern "a[b/c]d" does not match such pathnames as abd or a/d. It only matches a pathname of literally a[b/c]d.
2. If a filename begins with a <period> ( ‘’. ), the <period> shall be explicitly matched by using a <period> as the first character of the pattern or immediately following a <slash> character. The leading <period> shall not be matched by:
   • The <asterisk> or <question-mark> special characters
   • A bracket expression containing a non-matching list, such as "[!a]", a range expression, such as "[%-0]", or a character class expression, such as "[[:punct:]]"

   It is unspecified whether an explicit <period> in a bracket expression matching list, such as "[.abc]", can match a leading <period> in a filename.

3. Specified patterns shall be matched against existing filenames and pathnames, as appropriate. Each component that contains a pattern character shall require read permission in the directory containing that component. Any component, except the last, that does not contain a pattern character shall require search permission. For example, given the pattern:

   /foo/bar/x*/bam

   search permission is needed for directories / and foo, search and read permissions are needed for directory bar, and search permission is needed for each x* directory. If the pattern matches any existing filenames or pathnames, the pattern shall be replaced with those filenames and pathnames, sorted according to the collating sequence in effect in the current locale. If this collating sequence does not have a total ordering of all characters (see XBD V1_chap07(7)), any filenames or pathnames that collate equally should be further compared byte-by-byte using the collating sequence for the POSIX locale.

   Note:

   A future version of this standard may require the byte-by-byte further comparison described above.

   If the pattern contains an open bracket ( [‘’ ) that does not introduce a bracket expression as in XBD V1_chap09(7), it is unspecified whether other unquoted pattern matching characters within the same slash-delimited component of the pattern retain their special meanings or are treated as ordinary characters. For example, the pattern "a*[/b*" may match all filenames beginning with ‘b’ in the directory "a*[" or it may match all filenames beginning with ‘b’ in all directories with names beginning with ‘a’ and ending with [‘’.

   If the pattern does not match any existing filenames or pathnames, the pattern string shall be left unchanged.

2.14. Special Built-In Utilities

The following "special built-in" utilities shall be supported in the shell command language. The output of each command, if any, shall be written to standard output, subject to the normal redirection and piping possible with all commands.

The term "built-in" implies that the shell can execute the utility directly and does not need to search for it. An implementation may choose to make any utility a built-in; however, the special built-in utilities described here differ from regular built-in utilities in two respects:

1. An error in a special built-in utility may cause a shell executing that utility to abort, while an error in a regular built-in utility shall not cause a shell executing that utility to abort. (See Consequences of Shell Errors for the consequences of errors on interactive and non-interactive shells.) If a special built-in utility encountering an error does not abort the shell, its exit value shall be non-zero.
2. As described in Simple Commands, variable assignments preceding the invocation of a special built-in utility remain in effect after the built-in completes; this shall not be the case with a regular built-in or other utility.

The special built-in utilities in this section need not be provided in a manner accessible via the exec family of functions defined in the System Interfaces volume of POSIX.1-2008.

Some of the special built-ins are described as conforming to XBD V1_chap12(7). For those that are not, the requirement in Utility Description Defaults that "--" be recognized as a first argument to be discarded does not apply and a conforming application shall not use that argument.