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       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 .