CAT(1) General Commands Manual CAT(1)

cat — concatenate and print files

cat [-u] [file ...]

The cat utility shall read files in sequence and shall write their contents to the standard output in the same sequence.

The cat utility shall conform to XBD V1_chap12(7).

The following option shall be supported:

Write bytes from the input file to the standard output without delay as each is read.

The following operand shall be supported:

file
A pathname of an input file. If no file operands are specified, the standard input shall be used. If a file is '-' , the cat utility shall read from the standard input at that point in the sequence. The cat utility shall not close and reopen standard input when it is referenced in this way, but shall accept multiple occurrences of '-' as a file operand.

The standard input shall be used only if no file operands are specified, or if a file operand is ‘-’. See the INPUT FILES section.

The input files can be any file type.

The following environment variables shall affect the execution of cat:

Provide a default value for the internationalization variables that are unset or null. (See XBD for the precedence of internationalization variables used to determine the values of locale categories.)
If set to a non-empty string value, override the values of all the other internationalization variables.
Determine the locale for the interpretation of sequences of bytes of text data as characters (for example, single-byte as opposed to multi-byte characters in arguments).

Determine the locale that should be used to affect the format and contents of diagnostic messages written to standard error.
[ XSI] Determine the location of messages objects and message catalogs.

Default.

The standard output shall contain the sequence of bytes read from the input files. Nothing else shall be written to the standard output. If the standard output is a regular file, and is the same file as any of the input file operands, the implementation may treat this as an error.

The standard error shall be used only for diagnostic messages.

None.

None.

The following exit values shall be returned:

All input files were output successfully.
An error occurred.

Default.

The -u option has value in prototyping non-blocking 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-defined whether the cat utility buffers output if the -u option is not specified. Traditionally, the -u option is implemented using the equivalent of the setvbuf(3) function defined in the System Interfaces volume of POSIX.1-2024.

The following command:

cat myfile

writes the contents of the file to standard output.

The following command:

cat doc1 doc2 > doc.all

concatenates the files and and writes the result to .

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 before cat even begins execution. This is true whether the cat command fails with an error or silently succeeds (the specification allows both behaviors). In order to append the contents of without losing the original contents of doc, this command should be used instead:

cat doc.end >> doc

The command:

cat start - middle - end > file

when standard input is a terminal, gets 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.

Historical versions of the cat utility include the -e, -t, and -v, options which permit the ends of lines, <tab> characters, and invisible characters, respectively, to be rendered visible in the output. The standard developers 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 l pathname

The latter also has the advantage that its output is unambiguous, whereas the output of historical cat -etv is not.

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 can be accomplished by the shell script shown in the following example:

sed -n '
# Write non-empty lines.
/./   {
      p
      d
      }
# Write a single empty line, then look for more empty lines.
/^$/  p
# Get next line, discard the held <newline> (empty line),
# and look for more empty lines.
:Empty
/^$/  {
      N
      s/.//
      b Empty
      }
# Write the non-empty line before going back to search
# for the first in a set of empty lines.
      p
'

The System V -s option to silence error messages can be accomplished by redirecting the standard error. 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 was omitted because similar functionality can be obtained from the -n option of the pr(1) utility.

None.

more(1)

XBD V1_chap08(7), V1_chap12(7)

XSH setvbuf(3)

First released in Issue 2.

SD5-XCU-ERN-97 is applied, updating the SYNOPSIS.

SD5-XCU-ERN-174 is applied, changing the RATIONALE.

POSIX.1-2008, Technical Corrigendum 2, XCU/TC2-2008/0073 [876] is applied.

Austin Group Defect 1122 is applied, changing the description of NLSPATH .

June 14, 2024 posix.fail