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cat — 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, Section 12.2
, 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 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
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 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 LC_MESSAGES.
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.
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 cat
utility buffers output if the
option is not specified. Traditionally, the −u
option is implemented using the equivalent of the setvbuf
defined in the System Interfaces volume of POSIX.1‐2008.
The following command:
writes the contents of the file myfile
to standard output.
The following command:
concatenates the files doc1
and writes the result to
Because of the shell language mechanism used to perform output redirection, a
command such as this:
causes the original data in doc
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 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
first time '−'
was used as a file
operand and an
end-of-file condition would be detected immediately when '−'
referenced the second time.
Historical versions of the cat
utility include the −e
, 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:
The latter also has the advantage that its output is unambiguous, whereas the
output of historical cat −etv
option was omitted because it corresponds to different
functions in BSD and System V-based systems. The BSD −s
to squeeze blank lines can be accomplished by the shell script shown in the
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 −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
The Base Definitions volume of POSIX.1‐2008, Chapter 8
, Section 12.2
, Utility Syntax
The System Interfaces volume of POSIX.1‐2008, setvbuf()
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 http://www.unix.org/online.html .
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