Section: POSIX Programmer's Manual (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.
--- concatenate and print files
cat [-u] [file...]
utility shall read files in sequence and shall write their contents
to the standard output in the same sequence.
utility shall conform to the Base Definitions volume of POSIX.1-2008,
, Utility Syntax Guidelines.
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:
A pathname of an input file. If no
operands are specified, the standard input shall be used. If a
utility shall read from the standard input at that point in the
utility shall not close and reopen standard input when it is referenced
in this way, but shall accept multiple occurrences of
The standard input shall be used only if no
operands are specified, or if a
See the INPUT FILES section.
The input files can be any file type.
The following environment variables shall affect the execution of
Provide a default value for the internationalization variables that are
unset or null. (See the Base Definitions volume of POSIX.1-2008,
Section 8.2, Internationalization Variables
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.
Determine the location of message catalogs for the processing of
The standard output shall contain the sequence of bytes read from the
input files. Nothing else shall be written to the standard output.
The standard error shall be used only for diagnostic messages.
The following exit values shall be returned:
All input files were output successfully.
An error occurred.
CONSEQUENCES OF ERRORS
The following sections are informative.
option has value in prototyping non-blocking reads from FIFOs. The
intent is to support the following sequence:
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
option guarantees that unbuffered I/O is available. It is
implementation-defined whether the
utility buffers output if the
option is not specified. Traditionally, the
option is implemented using the equivalent of the
function defined in the System Interfaces volume of POSIX.1-2008.
The following command:
writes the contents of the file
to standard output.
The following command:
cat doc1 doc2 > doc.all
concatenates the files
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
to be lost.
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
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
the first time
was used as a
operand and an end-of-file condition would be detected immediately when
was referenced the second time.
Historical versions of the
utility include the
options which permit the ends of lines,
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
option was omitted because it corresponds to different functions in BSD
and System V-based systems. The BSD
option to squeeze blank lines can be accomplished by the shell script
shown in the following example:
sed -n '
# Write non-empty lines.
# Write a single empty line, then look for more empty lines.
# Get next line, discard the held <newline> (empty line),
# and look for more empty lines.
# Write the non-empty line before going back to search
# for the first in a set of empty lines.
The System V
option to silence error messages can be accomplished by redirecting the
standard error. Note that the BSD documentation for
uses the term ``blank line'' to mean the same as the POSIX ``empty
line'': a line consisting only of a
option was omitted because similar functionality can be obtained from
option of the
The Base Definitions volume of POSIX.1-2008,
Chapter 8, Environment Variables,
Section 12.2, Utility Syntax Guidelines
The System Interfaces volume of POSIX.1-2008,
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, Copyright (C) 2013 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group.
(This is POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) 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
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