command is a standard Unix
program used to concatenate
and display files. The name is from catenate
, a synonym of concatenate.
The Single Unix Specification
specifies the behavior that each of the files given in sequence as arguments will write their contents to the standard output in the same sequence, and mandates one option, -u
, where each byte is printed as it is read.
If the filename is specified as -, then cat will read from standard input at that point in the sequence. If no files are specified, cat will read from standard input.
Both the BSD versions of cat
(as per the OpenBSD
manpage) and the GNU coreutils
version of cat
specify the following options:
- -b (GNU only: --number-nonblank), number non-blank output lines
- -n (GNU only: --number), number all output lines
- -s (GNU only: --squeeze-blank), squeeze multiple adjacent blank lines
- -v (GNU only: --show-nonprinting), displays nonprinting characters as if they were visible, except for tabs and the end of line character
- -t on BSD, -T on GNU, implies -v but also display tabs as ^I
- -e on BSD, -E on GNU, implies -v but also display end-of-line characters as $
Jargon File definition
The Jargon File
version 4.4.7 lists this as the definition of cat
Useless use of cat
UUOC (from comp.unix.shell on Usenet) stands for "Useless Use of cat". As received wisdom on comp.unix.shell observes, "The purpose of cat is to concatenate (or 'catenate') files. If it's only one file, concatenating it with nothing at all is a waste of time, and costs you a process." Nevertheless one sees people doing
cat file | some_command and its args ...
instead of the equivalent and cheaper
<file some_command and its args ...
or (equivalently and more classically)
some_command and its args ... <file
Since 1995, occasional awards for UUOC have been given out, usually by Perl luminary Randal L. Schwartz. There is a web page devoted to this and other similar awards. In British hackerdom the activity of fixing instances of UUOC is sometimes called demoggification.
Amongst the mildly paranoid it is still considered safer to use cat for such cases given that the < and > keys are next to each other in many popular keyboard mappings. While the risk might be low, the impact of using > instead of < can be high and prohibitive.
is a UNIX
program similar to cat
, that decompresses individual files and concatenates
them to standard output. Traditionally zcat
operated on files compressed by compress
but today it is usually able to operate on gzip
or even bzip2
archives. On such systems, it's equivalent to gunzip -c