<code>cat</code> is a shell command for writing the content of a file or input stream to standard output. The name is an abbreviation of , a variant form of concatenate. Originally developed for Unix, it is available on many operating systems and shells today.
In addition to combining files, <code>cat</code> is commonly used to copy files and in particular to copy a file to the terminal monitor. Unless redirected, outputs file content on-screen.
<code>cat</code> was part of the early versions of Unix, e.g., Version 1. It replaced <code>pr</code>, a PDP-7 and Multics command for copying a single file to the screen. It was written by Ken Thompson and Dennis Ritchie. The implementation of <code>cat</code> bundled in GNU coreutils was written by Torbjorn Granlund and Richard Stallman.
The <code>cat</code> command can be used to serve various needs including concatenation and display. A use case for concatenation is to specify multiple input files and to redirect output to another file to persist the result. A common use case for display is to specify a single file without redirecting output so that the file content displays on the monitor.
The Single Unix Specification defines the operation of <code>cat</code> to read files in the sequence given in its arguments, writing their contents to the standard output in the same sequence. The specification mandates the support of one option flag, for unbuffered output, meaning that each byte is written after it has been read. Some implementations, like GNU Core Utilities, do this by default and ignore the flag.
If one of the input filenames is specified as a single hyphen (-), then <code>cat</code> reads from standard input at that point in the sequence. If no files are specified, <code>cat</code> reads from standard input only.
The command-syntax is: cat [options] [path...]
Options include:
<code>cat</code> can be used to pipe a file to a program that expects plain text or binary data on its input stream. <code>cat</code> does not destroy non-text bytes when concatenating and outputting. As such, its two main use cases are text files and certain format-compatible types of binary files.
Concatenation of text is limited to text files using the same legacy encoding, such as ASCII. <code>cat</code> does not provide a way to concatenate Unicode text files that have a Byte Order Mark or files using different text encodings from each other.
For many structured binary data sets, the resulting combined file may not be valid; for example, if a file has a unique header or footer, the result will spuriously duplicate these. However, for some multimedia digital container formats, the resulting file is valid, and so <code>cat</code> provides an effective means of appending files. Video streams can be a significant example of files that <code>cat</code> can concatenate without issue, e.g. the MPEG program stream (MPEG-1 and MPEG-2) and DV (Digital Video) formats, which are fundamentally simple streams of packets.
The Jargon File version 4.4.7 lists this as the definition of <code>cat</code>:
Useless use of cat (UUOC) is common Unix jargon for command line constructs that provide only a function of convenience to the user. In computing, the word "abuse", in the sense of "improper or excessive use", is used to disparage the excessive or unnecessary use of a language construct; thus, abuse of is sometimes called "cat abuse". Example of a common abuse is:
cat filename | command arg1 arg2 argn
This can be rewritten using redirection of stdin instead, in either of the following forms (the first is more traditional):
command arg1 arg2 argn < filename
<filename command arg1 arg2 argn
Beyond other benefits, the input redirection forms allow command to perform random access on the file, whereas the examples do not. This is because the redirection form opens the file as the stdin file descriptor which command can fully access, while the form simply provides the data as a stream of bytes.
Another common case where is unnecessary is where a command defaults to operating on stdin, but will read from a file, if the filename is given as an argument. This is the case for many common commands; the following examples
cat file | grep pattern
cat file | less
can instead be written as
grep pattern file
less file
A common interactive use of for a single file is to output the content of a file to standard output. However, if the output is piped or redirected, is unnecessary.
A written with UUOC might still be preferred for readability reasons, as reading a piped stream left-to-right might be easier to conceptualize. Also, one wrong use of the redirection symbol instead of (often adjacent on keyboards) may permanently delete the content of a file, in other words clobbering, and one way to avoid this is to use with pipes. Compare:
command < in | command2 > out
<in command | command2 > out
with: cat in | command | command2 > out