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.
cat — concatenate and print files
cat [-u] [file...]
The cat utility shall read files in sequence and shall write their contents to the standard output in the same sequence.
The cat utility shall conform to the Base Definitions volume of POSIX.1-2017, 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 operand.
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.1-2017, 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. 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.
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.
The −u 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 −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.
The following command:
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
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.
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.
# 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 utility.
The Base Definitions volume of POSIX.1-2017, Chapter 8, Environment Variables, Section 12.2, Utility Syntax Guidelines
The System Interfaces volume of POSIX.1-2017, setvbuf()
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 .
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