SH(1) General Commands Manual SH(1)

sh — shell, the standard command language interpreter

sh [−aCefinuvx ] [ command_file [argument . . . ] ]
sh −c [−aCefinuvx ] command_string [ command_name [argument . . . ] ]
sh −s [−aCefinuvx ] [argument . . . ]

The sh utility is a command language interpreter that shall execute commands read from a command-line string, the standard input, or a specified file. The commands to be executed shall be expressed in the language described in Section 3.

The sh utility shall conform to the utility argument syntax guidelines described in 2.10.2. The −a, −C, −e, −f, −n, −u, −v, and −x options are described as part of the set utility in set(1) . The following additional options shall be supported by the implementation:

Read commands from the command_string operand. Set the value of special parameter 0 (see 3.5.2) from the value of the command_name operand and the positional parameters ($1, $2, etc.) in sequence from the remaining argument operands. No commands shall be read from the standard input.

Specify that the shell is interactive; see below. An implementation may treat specifying the −i option as an error if the real user ID of the calling process does not equal the effective user ID or if the real group ID does not equal the effective user ID.

Read commands from the standard input. If there are no operands and the −c option is not specified, the −s option shall be assumed. If the −i option is present, or if there are no operands and the shell’s standard input and standard error are attached to a terminal, the shell is considered to be interactive. (See 3.1.4.) The behavior of an interactive shell is not fully specified by this standard.

: The preceding sentence is expected to change following the eventual approval of the UPE supplement.

Implementations may accept the option letters with a leading plus sign (+) instead of a leading hyphen (meaning the reverse case of the option as described in this standard). A conforming application shall protect its first operand, if it starts with a plus sign, by preceding it with the − − argument that denotes ‘‘end of options.’’

The following operands shall be supported by the implementation:

  • A single hyphen shall be treated as the first operand and then ignored. If both - and - - are given as arguments, or if other operands precede the single hyphen, the results are undefined. argument The positional parameters ($1, $2, etc.) shall be set to arguments, if any. command_file The pathname of a file containing commands. If the pathname contains one or more slash characters, the implementation shall attempt to read that file; the file need not be executable. If the pathname does not contain a slash character:
  • The implementation shall attempt to read that file from the current working directory; the file need not be executable.
  • If the file is not in the current working directory, the implementation may perform a search for an executable file using the value of PATH, as described in 3.9.1.1. Special parameter 0 (see 3.5.2) shall be set to the value of command_file. If sh is called using a synopsis form that omits command_file, special parameter 0 shall be set to the value of the first argument passed to sh from its parent (e.g., argv[0] in the C binding), which is normally a pathname used to execute the sh utility. command_name A string assigned to special parameter 0 when executing the commands in command_string. If command_name is not specified, special parameter 0 shall be set to the value of the first argument passed to sh from its parent (e.g., argv[0] in the C binding), which is normally a pathname used to execute the sh utility. command_string A string that shall be interpreted by the shell as one or more commands, as if the string were the argument to the function in 7.1.1 [such as the () function in the C binding]. If the command_string operand is an empty string, sh shall exit with a

zero exit status.

The standard input shall be used only if:

  1. The −s option is specified, or;
  2. The −c option is not specified and no operands are specified, or;
  3. The script executes one or more commands that require input from standard input (such as a read command that does not redirect its input).

See Input Files. When the shell is using standard input and it invokes a command that also uses standard input, the shell shall ensure that the standard input file pointer points directly after the command it has read when the command begins execution. It shall not read ahead in such a manner that any characters intended to be read by the invoked command are consumed by the shell (whether interpreted by the shell or not) or that characters that are not read by the invoked command are not seen by the shell. When the command expecting to read standard input is started asynchronously by an interactive shell, it is unspecified whether characters are read by the command or interpreted by the shell. If the standard input to sh is a or terminal device and is set to nonblocking reads, then sh shall enable blocking reads on standard input. This shall remain in effect when the command completes.

The input file shall be a text file, except that line lengths shall be unlimited. If the input file is empty or consists solely of blank lines and/or comments, sh shall exit with a zero exit status.

The following environment variables shall affect the execution of sh:

This variable shall be interpreted as the pathname of the user’s home directory. The contents of HOME are used in Tilde Expansion as described in 3.6.1.

Input field separators: a string treated as a list of characters that shall be used for field splitting and to split lines into words with the read command. See 3.6.5. If IFS is not set, the shell shall behave as if the value of IFS were the <space>, <tab>, and <newline> characters. Implementations may ignore the value of IFS in the

environment at the time sh is invoked, treating IFS as if it were not set.

This variable shall determine the locale to use for the locale categories when both LC_ALL and the corresponding environment variable (beginning with LC_ ) do not specify a locale. See 2.6.

This variable shall determine the locale to be used to override any values for locale categories specified by the settings of LANG or any environment variables beginning with LC_.

This variable shall determine the behavior of range expressions, equivalence classes, and multicharacter collating elements within pattern matching.

This variable shall determine the locale for the interpretation of sequences of bytes of text data as characters (e.g., single- versus multibyte characters in arguments and input files), which characters are defined as letters (character class alpha), and the behavior of character classes within pattern matching.

This variable shall determine the language in which messages should be written.

This variable shall represent a string formatted as described in 2.6, used to effect command interpretation. See 3.9.1.1.

Default.

See Standard Error.

Except as otherwise stated (by the descriptions of any invoked utilities or in interactive mode), standard error is used only for diagnostic messages.

None.

See Section 3.

The sh utility shall exit with one of the following values:

The script to be executed consisted solely of zero or more blank lines and/or comments.

1−125 A noninteractive shell detected a syntax, redirection, or variable assignment error.

A specified command_file could not be found by a noninteractive shell.

Otherwise, the shell shall return the exit status of the last command it invoked or attempted to invoke (see also the exit utility in exit(1) ).

See 3.8.1.

sh -c "cat myfile" sh my_shell_cmds The sh utility and the set special built-in utility share a common set of options. Unlike set, however, the POSIX. 2 sh does not specify the use of + as an option flag, because it is not particularly useful (the + variety generally invokes the default behavior) and because () does not support it. However, since many historical implementations do support the plus, applications will have to guard against the relatively obscure case of a first operand with a leading plus sign. There is a large number of environment variables used by historical implementations of sh that will not be introduced by POSIX. 2 until the UPE is completed. The KornShell ignores the contents of IFS upon entry to the script. A conforming application cannot rely on importing IFS. One justification for this, beyond security considerations, is to assist possible future shell compilers. Allowing IFS to be imported from the environment will prevent many optimizations that might otherwise be performed via dataflow analysis of the script itself.

The standard input and standard error are the files that determine whether a shell is interactive when −i is not specified. For example, sh > file and sh 2> file create interactive and noninteractive shells, respectively. Although both accept terminal input, the results of error conditions will be different, as described in 3.8.1; in the second example a redirection error encountered by a special built-in utility will abort the shell. The text in Standard Input about nonblocking reads concerns an instance of sh that has been invoked, probably by a C-language program, with standard input that has been opened using the flag; see POSIX. 1 {8} (). If the shell did not reset this flag, it would immediately terminate because no input data would be available yet and that would be considered the same as end-of-file.

See the Rationale for Section 3 concerning the lack of interactive features in sh. These features, including optional job control, are scheduled to be added in the User Portability Extension. The and variables are not specified because this standard, without UPE, does not describe an interactive shell. The options associated with a restricted shell (command name rsh and the −r option) were excluded because the developers of the standard felt that the implied level of security was not achievable and they did not want to raise false expectations. On systems that support set-user-ID scripts, a historical trapdoor has been to link a script to the name −i. When it is called by a sequence such as sh - or by #! /bin/ sh - the historical systems have assumed that no option letters follow. Thus, POSIX. 2 allows the single hyphen to mark the end of the options, in addition to the use of the regular - - argument, because it was felt that the older practice was so pervasive. An alternative approach is taken by the KornShell, where real and effective user/group IDs must match for an interactive shell; this behavior is specifically allowed by POSIX. 2. (Note: there are other problems with set-user-ID scripts that the two approaches described here do not deal with.)

September 1991 posix.fail