STANDARD INPUT

The standard input is used only if no file operands are specified, or if a file operand is −. See Input Files.

INPUT FILES

The input files can be any file type.

ENVIRONMENT VARIABLES

The following environment variables shall affect the execution of cat:

LANG

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.

LC_ALL

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_.

LC_CTYPE

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).

LC_MESSAGES

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

ASYNCHRONOUS EVENTS

Default.

STANDARD OUTPUT

The standard output shall contain the sequence of bytes read from the input file(s). Nothing else shall be written to the standard output.

STANDARD ERROR

Used only for diagnostic messages.

OUTPUT FILES

None.

EXAMPLES

Historical versions of the cat utility include the options −e, −t, and −v, which permit the ends of lines, <tab>s, and invisible characters, respectively, to be rendered visible in the output. The working group omitted these options because they provide too fine a degree of control over what is made visible, and similar output can be obtained using a command such as: sed -n -e ’s/$/$/’ -e l pathname The −s option was omitted because it corresponds to different functions in BSD and System V-based systems. The BSD −s option to squeeze blank lines will be handled by more -s in the UPE. The System V −s option to silence error messages can be accomplished by redirecting the standard error. An alternative to cat −s is the following shell script using sed:

sed -n ’ # Write non-empty lines. /./ { p d } # Write a single empty line, then look for more empty lines. /ˆ$/ p # Get next line, discard the held <newline> (empty line), # and look for more empty lines. :Empty /ˆ$/ { N s/.// b Empty } # Write the non-empty line before going back to search # for the first in a set of empty lines. p ’ Note that the BSD documentation for cat uses the term ‘‘blank line’’ to mean the same as the POSIX ‘‘empty line’’; a line consisting only of a <newline>. The BSD −n option is omitted because similar functionality can be obtained from the −n option of the pr utility. The −u option is included here for its value in prototyping nonblocking reads from FIFOs. The intent is to support the following sequence: mkfifo foo cat −u foo > /dev/tty13 & cat −u > foo It is unspecified whether standard output is or is not buffered in the default case. This is sometimes of interest when standard output is associated with a terminal, since buffering may delay the output. The presence of the −u option guarantees that unbuffered I/O is available. It is implementation dependent whether the cat utility buffers output if the −u option is not specified. Traditionally, the −u option is implemented using the BSD setbuffer() function, the System V setbuf() function, or the C Standard {7} setvbuf() function. The following command cat myfile writes the contents of the file myfile to standard output. The following command cat doc1 doc2 > doc.all concatenates the files doc1 and doc2 and writes the result to doc.all. Because of the shell language mechanism used to perform output redirection, a command such as this:

cat doc doc.end > doc causes the original data in doc to be lost. Due to changes made to subclause 2.11.4 in Draft 11, the description of the file operand now states that − must be accepted multiple times, as in historical practice. This allows the command: cat start - middle - end > file when standard input is a terminal, to get two arbitrary pieces of input from the terminal with a single invocation of cat. Note, however, that if standard input is a regular file, this would be equivalent to the command: cat start - middle /dev/null end > file because the entire contents of the file would be consumed by cat the first time − was used as a file operand and an end-of-file condition would be detected immediately when − was referenced the second time.

HISTORY OF DECISIONS MADE

None.