Cat (Unix)

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This article includes a list of references, related reading or external links, but its sources remain unclear because it lacks inline citations. Please improve this article by introducing more precise citations where appropriate. (February 2008)

The cat command is a standard Unix program used to concatenate and display files. The name is from catenate, a synonym of concatenate.

Specification

The Single Unix Specification specifies the behavior that the contents of each of the files given in sequence as arguments will be written 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 entered.

Extensions

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 $

Unix culture

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

Without two named files cat is a useless use of cat. A UUOC campaign will eliminate the inefficiency from shell scripts by using redirection instead. But cat with one named file is safer where human error is a concern— one wrong use of the default ^[1] redirection symbol ">" instead of "<" (often adjacent on keyboards) will permanently delete ^[2] the file you were just needing to read.

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^[3].

Other operating systems

The equivalent command in the VMS, CP/M, DOS, OS/2, and Microsoft Windows operating system command shells is type.

See also

• Coreutils
• List of Unix utilities
• split, a command that splits a file into pieces which cat can then rejoin.
• tac, a similar tool which concatenates files backwards.
• zcat
• paste (Unix)

External links

• cat: compare two files – Commands & Utilities Reference, The Single UNIX® Specification, Issue 7 from The Open Group

Manual pages

• cat(1) original manual page in the First Edition of Unix.
• cat(1): concatenate and write files – GNU Coreutils reference
• cat(1): concatenate and print files – OpenBSD General Commands Manual
• cat(1) – FreeBSD General Commands Manual
• cat(1) Manual page from Plan 9 from Bell Labs.

Other

• UUOC awards

References

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