This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface

       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.

       Default.

       Default.

       Thefollowingsectionsareinformative.

       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(1POSIX)

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

       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, Section8.2, InternationalizationVariables 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.

       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.

       The following exit values shall be returned:

        0    All input files were output successfully.

       >0    An error occurred.

       None.

       None.

       The input files can be any file type.

       cat — concatenate and print files

       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  cat  utility  shall  conform  to  the Base Definitions volume of POSIX.1‐2017, Section12.2, UtilitySyntaxGuidelines.

       The following option shall be supported:

       -u        Write bytes from the input file to the standard output without delay as each is read.

       None.

       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.

       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.

more

       The  Base  Definitions  volume  of  POSIX.1‐2017, Chapter8, EnvironmentVariables, Section12.2, UtilitySyntaxGuidelines

       The System Interfaces volume of POSIX.1‐2017, setvbuf()

       The standard error shall be used only for diagnostic messages.

       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 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.

       cat [-u][file...]