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PROLOG | NAME | SYNOPSIS | DESCRIPTION | OPTIONS | OPERANDS | STDIN | INPUT FILES | ENVIRONMENT VARIABLES | ASYNCHRONOUS EVENTS | STDOUT | STDERR | OUTPUT FILES | EXTENDED DESCRIPTION | EXIT STATUS | CONSEQUENCES OF ERRORS | APPLICATION USAGE | EXAMPLES | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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CAT(1P) POSIX Programmer's Manual CAT(1P)
This manual page is part of the POSIX Programmer's Manual. The
Linux implementation of this interface may differ (consult the
corresponding Linux manual page for details of Linux behavior), or
the interface may not be implemented on Linux.
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 the Base Definitions volume of
POSIX.1‐2017, Section 12.2, Utility Syntax Guidelines.
The following option shall be supported:
-u 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:
LANG Provide a default value for the internationalization
variables that are unset or null. (See the Base
Definitions volume of POSIX.1‐2017, Section 8.2,
Internationalization Variables for the precedence of
internationalization variables used to determine the
values of locale categories.)
LC_ALL If set to a non-empty string value, override the values
of all the other internationalization variables.
LC_CTYPE 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).
LC_MESSAGES
Determine the locale that should be used to affect the
format and contents of diagnostic messages written to
standard error.
NLSPATH Determine the location of message catalogs for the
processing of LC_MESSAGES.
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:
0 All input files were output successfully.
>0 An error occurred.
Default.
The following sections are informative.
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() function defined in the System Interfaces volume
of POSIX.1‐2017.
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 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 doc.end 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 utility.
None.
more(1p)
The Base Definitions volume of POSIX.1‐2017, Chapter 8,
Environment Variables, Section 12.2, Utility Syntax Guidelines
The System Interfaces volume of POSIX.1‐2017, setvbuf(3p)
Portions of this text are reprinted and reproduced in electronic
form from IEEE Std 1003.1-2017, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The
Open Group Base Specifications Issue 7, 2018 Edition, Copyright
(C) 2018 by the Institute of Electrical and Electronics Engineers,
Inc and The Open Group. In the event of any discrepancy between
this version and the original IEEE and The Open Group Standard,
the original IEEE and The Open Group Standard is the referee
document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .
Any typographical or formatting errors that appear in this page
are most likely to have been introduced during the conversion of
the source files to man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2017 CAT(1P)
Pages that refer to this page: tee(1p)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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