Ubuntu Manpage: cat - concatenate and print files

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 IEEE Std 1003.1-2001, 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 IEEE Std 1003.1-2001, 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
       IEEE Std 1003.1-2001.

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 options -e, -t, and -v, which permit the ends of
lines, <tab>s, 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 -e 's/$/$/' -e l pathname

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, 2003 Edition,
       Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open  Group  Base
       Specifications Issue 6, Copyright (C) 2001-2003 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 .

IEEE/The Open Group                                   2003                                                CAT(P)