cat
is a standard
Unix utility
Unix (; trademarked as UNIX) is a family of multitasking, multiuser computer operating systems that derive from the original AT&T Unix, whose development started in 1969 at the Bell Labs research center by Ken Thompson, Dennis Ritchie, and o ...
that reads files sequentially, writing them to
standard output
In computer programming, standard streams are interconnected input and output communication channels between a computer program and its environment when it begins execution. The three input/output (I/O) connections are called standard input (stdin ...
. The name is derived from its function to (con)catenate files (from Latin ''catenare'', "to chain"). It has been ported to a number of operating systems.
History
cat
was part of the early versions of
Unix
Unix (; trademarked as UNIX) is a family of multitasking, multiuser computer operating systems that derive from the original AT&T Unix, whose development started in 1969 at the Bell Labs research center by Ken Thompson, Dennis Ritchie, and ot ...
, e.g.,
Version 1
Version 1 is an Irish company specializing in international management consulting, software asset management, software development, cloud computing, and outsourcing company.
On July 13, 2022, Partners Group completed the acquisition of a maj ...
, and replaced
pr
, a
PDP-7
The PDP-7 was a minicomputer produced by Digital Equipment Corporation as part of the PDP series. Introduced in 1964, shipped since 1965, it was the first to use their Flip-Chip technology. With a cost of , it was cheap but powerful by the st ...
and
Multics
Multics ("Multiplexed Information and Computing Service") is an influential early time-sharing operating system based on the concept of a single-level memory.Dennis M. Ritchie, "The Evolution of the Unix Time-sharing System", Communications of t ...
utility for copying a single file to the screen.
It was written by
Ken Thompson
Kenneth Lane Thompson (born February 4, 1943) is an American pioneer of computer science. Thompson worked at Bell Labs for most of his career where he designed and implemented the original Unix operating system. He also invented the B programmi ...
and
Dennis Ritchie
Dennis MacAlistair Ritchie (September 9, 1941 – October 12, 2011) was an American computer scientist. He is most well-known for creating the C programming language and, with long-time colleague Ken Thompson, the Unix operating system and B p ...
.
The version of
cat
bundled in
GNU
GNU () is an extensive collection of free software (383 packages as of January 2022), which can be used as an operating system or can be used in parts with other operating systems. The use of the completed GNU tools led to the family of operat ...
coreutils
The GNU Core Utilities or coreutils is a package of GNU software containing implementations for many of the basic tools, such as cat, ls, and rm, which are used on Unix-like operating systems.
In September 2002, the ''GNU coreutils'' were cr ...
was written by Torbjorn Granlund and
Richard Stallman
Richard Matthew Stallman (; born March 16, 1953), also known by his initials, rms, is an American free software movement activist and programmer. He campaigns for software to be distributed in such a manner that its users have the freedom to ...
. The
ReactOS
ReactOS is a free and open-source operating system for amd64/i686 personal computers intended to be binary-compatible with computer programs and device drivers made for Windows Server 2003 and later versions of Windows. ReactOS has been noted a ...
version was written by David Welch, Semyon Novikov, and Hermès Bélusca.
Over time, alternative utilities such as
tac
and
bat
also became available, bringing different new features.
Usage
The
Single Unix Specification defines the operation of
cat
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, ''u'' for unbuffered output, meaning that each byte is written after it has been read. Some operating systems, like the ones using GNU
Core Utilities, do this by default and ignore the flag.
[GNU Coreutils]
"GNU Coreutils manual"
''GNU
GNU () is an extensive collection of free software (383 packages as of January 2022), which can be used as an operating system or can be used in parts with other operating systems. The use of the completed GNU tools led to the family of operat ...
'', Retrieved on 1 Mars 2017.
If one of the input filenames is specified as a single hyphen (''-''), then
cat
reads from standard input at that point in the sequence. If no files are specified,
cat
reads from standard input only.
The command-
syntax
In linguistics, syntax () is the study of how words and morphemes combine to form larger units such as phrases and sentences. Central concerns of syntax include word order, grammatical relations, hierarchical sentence structure ( constituency) ...
is:
cat
ptions ile_names
Options
Example of some
cat
options:
* (GNU: ), number non-blank output lines
* implies but also display end-of-line characters as (GNU only: the same, but without implying )
* (GNU: ), number all output lines
* (GNU: ), squeeze multiple adjacent blank lines
* implies , but also display tabs as (GNU: the same, but without implying )
* use unbuffered I/O for stdout. POSIX does not specify the behavior without this option.
* (GNU: ), displays nonprinting characters, except for tabs and the end of line character
Use cases
cat
can be used to pipe a file to a program that expects plain text or binary data on its input stream.
cat
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
ASCII ( ), abbreviated from American Standard Code for Information Interchange, is a character encoding standard for electronic communication. ASCII codes represent text in computers, telecommunications equipment, and other devices. Because of ...
.
cat
does not provide a way to concatenate Unicode text files that have a
Byte Order Mark
The byte order mark (BOM) is a particular usage of the special Unicode character, , whose appearance as a magic number at the start of a text stream can signal several things to a program reading the text:
* The byte order, or endianness, of th ...
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 format
A container format (informally, sometimes called a wrapper) or metafile is a file format that allows multiple data streams to be embedded into a single file, usually along with metadata for identifying and further detailing those streams. Notab ...
s, the resulting file is valid, and so
cat
provides an effective means of appending files. Video streams can be a significant example of files that
cat
can concatenate without issue, e.g. the
MPEG program stream
Program stream (PS or MPEG-PS) is a container format for multiplexing digital audio, video and more. The PS format is specified in MPEG-1 Part 1 (ISO/IEC 11172-1) and MPEG-2 Part 1, Systems (ISO/IEC standard 13818-1/ITU-T H.222.0). The MPEG-2 Pr ...
(MPEG-1 and MPEG-2) and
DV (Digital Video) formats, which are fundamentally simple streams of packets.
Examples
Unix culture
Jargon file definition
The
Jargon File
The Jargon File is a glossary and usage dictionary of slang used by computer programmers. The original Jargon File was a collection of terms from technical cultures such as the MIT AI Lab, the Stanford AI Lab (SAIL) and others of the old ARPANET A ...
version 4.4.7 lists this as the definition of
cat
:
Useless use of cat
''Useless use of cat'' (''UUOC'') is common Unix jargon for command line constructs that only provide a function of convenience to the user.
[comp.unix.shell]
"Early award example of UUOC (1994)"
'' comp.unix.shell via Google Groups'', Retrieved on 1 Mars 2017. In computing, the word "abuse", in the second sense of the definition, is used to disparage the excessive or unnecessary use of a language construct; thus, abuse of
cat
is sometimes called "cat abuse". Example of a common
cat
abuse is given in the award:
cat filename , command arg1 arg2 argn
This can be rewritten using
redirection of
stdin
In computer programming, standard streams are interconnected input and output communication channels between a computer program and its environment when it begins execution. The three input/output (I/O) connections are called standard input (stdin ...
instead, in either of the following forms (the first is more traditional):
command arg1 arg2 argn < filename
Beyond other benefits, the input redirection forms allow ''command'' to perform random access
Random access (more precisely and more generally called direct access) is the ability to access an arbitrary element of a sequence in equal time or any datum from a population of addressable elements roughly as easily and efficiently as any othe ...
on the file, whereas the cat
examples do not. This is because the redirection form opens the file as the stdin file descriptor which ''command'' can fully access, while the cat
form simply provides the data as a stream of bytes.
Another common case where cat
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 cat
for a single file is to output the content of a file to standard output. However, if the output is piped or redirected, cat
is unnecessary.
A cat
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
In software engineering and computer science, clobbering a file, processor register or a region of computer memory is the process of overwriting its contents completely, whether intentionally or unintentionally, or to indicate that such an acti ...
, and one way to avoid this is to use cat
with pipes. Compare:
command < in , command2 > out
out
with:
cat in , command , command2 > out
See also
* paste
* split
Split(s) or The Split may refer to:
Places
* Split, Croatia, the largest coastal city in Croatia
* Split Island, Canada, an island in the Hudson Bay
* Split Island, Falkland Islands
* Split Island, Fiji, better known as Hạfliua
Arts, enterta ...
, a command that splits a file into pieces which cat can then rejoin.
* zcat
* less
Less or LESS may refer to: fewer than,: not as much.
Computing
* less (Unix), a Unix utility program
* Less (stylesheet language), a dynamic stylesheet language
* Large-Scale Scrum (LeSS), a product development framework that extends Scrum
Othe ...
References
External links
*
UNIX Style, or cat -v Considered Harmful
- A paper by Rob Pike on proper Unix command design using cat as an example.
cat(1) original manual page
in the First Edition of Unix
Unix (; trademarked as UNIX) is a family of multitasking, multiuser computer operating systems that derive from the original AT&T Unix, whose development started in 1969 at the Bell Labs research center by Ken Thompson, Dennis Ritchie, and ot ...
.
*
*
*
*
*
{{Core Utilities commands
Unix text processing utilities
Cat
The cat (''Felis catus'') is a domestic species of small carnivorous mammal. It is the only domesticated species in the family Felidae and is commonly referred to as the domestic cat or house cat to distinguish it from the wild members of ...
Unix SUS2008 utilities
IBM i Qshell commands