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PROLOG

       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.

NAME

       cat — concatenate and print files

SYNOPSIS

       cat [−u] [file...]

DESCRIPTION

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

OPTIONS

       The cat utility shall conform to the Base Definitions  volume  of  POSIX.1‐2008,  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.

OPERANDS

       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.

STDIN

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

INPUT FILES

       The input files can be any file type.

ENVIRONMENT VARIABLES

       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.1‐2008, 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.

ASYNCHRONOUS EVENTS

       Default.

STDOUT

       The  standard output shall contain the sequence of bytes read from the input files. Nothing else shall be
       written to the standard output.

STDERR

       The standard error shall be used only for diagnostic messages.

OUTPUT FILES

       None.

EXTENDED DESCRIPTION

       None.

EXIT STATUS

       The following exit values shall be returned:

        0    All input files were output successfully.

       >0    An error occurred.

CONSEQUENCES OF ERRORS

       Default.

       The following sections are informative.

APPLICATION USAGE

       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.1‐2008.

EXAMPLES

       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.

       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.

RATIONALE

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.

FUTURE DIRECTIONS

       None.

COPYRIGHT

       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 .

       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 .