cat (Unix)
GPLv2+ |
cat
is a standard
The other primary purpose of cat
, aside from concatenation, is file printing — allowing the computer user to view the contents of a file. Printing to files and the terminal are the most common uses of cat
.[3]
History
cat
was part of the early versions of
pr
, a PDP-7 and Multics utility for copying a single file to the screen.[4] It was written by Ken Thompson and Dennis Ritchiecat
bundled in Over time, alternative utilities such as tac
and bat
also became available, bringing different new features.[7][8]
Usage
The cat
utility serves a dual purpose: concatenating and printing.
With a single argument, it is often used to print a file to the user's
The
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 is:
cat [options] [file_names]
Options
Example of some cat
options:[11]
- -b (GNU: --number-nonblank), number non-blank output lines
- -e implies -v but also display end-of-line characters as $ (GNU only: -E the same, but without implying -v)
- -n (GNU: --number), number all output lines
- -s (GNU: --squeeze-blank), squeeze multiple adjacent blank lines
- -t implies -v, but also display tabs as ^I (GNU: -T the same, but without implying -v)
- -u use unbuffered I/O for stdout. POSIX does not specify the behavior without this option.
- -v (GNU: --show-nonprinting), 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
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
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 (MPEG-1 and MPEG-2) and DVExamples
Command | Explanation |
---|---|
cat file1.txt | Display contents of file |
cat file1.txt file2.txt | Concatenate two text files and display the result in the terminal |
cat file1.txt file2.txt > newcombinedfile.txt | Concatenate two text files and write them to a new file |
cat >newfile.txt | Create a file called newfile.txt. Type the desired input and press CTRL+D to finish. The text will be in file newfile.txt. |
cat -n file1.txt file2.txt > newnumberedfile.txt | Some implementations of cat, with option -n, can also number lines |
cat file1.txt > file2.txt | Copy the contents of file1.txt into file2.txt |
cat file1.txt >> file2.txt | Append the contents of file1.txt to file2.txt |
cat file1.txt file2.txt file3.txt | sort > test4 | Concatenate the files, sort the complete set of lines, and write the output to a newly created file |
cat file1.txt file2.txt | less | Run the program "less" with the concatenation of file1 and file2 as its input |
cat file1.txt | grep example | Highlight instances the word "example" in file1.txt |
command | cat | Cancel "command" special behavior (e.g. paging) when it writes directly to TTY (cf. UUOC below) |
Unix culture
Jargon file definition
The Jargon File version 4.4.7 lists this as the definition of cat
:
- To spew an entire file to the screen or some other output sink without pause (syn. blast).
- By extension, to dump large amounts of data at an unprepared target or with no intention of browsing it carefully. Usage: considered silly. Rare outside Unix sites. See also dd, BLT.
Among Unix fans, cat(1) is considered an excellent example of user-interface design, because it delivers the file contents without such verbosity as spacing or headers between the files, and because it does not require the files to consist of lines of text, but works with any sort of data.
Among Unix critics, cat(1) is considered the canonical example of bad user-interface design, because of its woefully unobvious name. It is far more often used to blast a single file to
LISP's cdr.[citation needed]
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.[12] In computing, the word "abuse",[13] 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
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 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.[14] 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 cat with pipes. Compare:
command < in | command2 > out
<in command | command2 > out
with:
cat in | command | command2 > out
See also
References
- ^ "In Unix, what do some obscurely named commands stand for?". University Information Technology Services. Indiana University.
- ^ Kernighan, Brian W.; Pike, Rob (1984). The UNIX Programming Environment. Addison-Wesley. p. 15.
- ^ a b Pike, Rob; Kernighan, Brian W. Program design in the UNIX environment (PDF) (Report). p. 3.
- McIlroy, M. D. (1987). A Research Unix reader: annotated excerpts from the Programmer's Manual, 1971–1986(PDF) (Technical report). CSTR. Bell Labs. 139.
- ^ Linux User Commands Manual –
- ^ "reactos/cat.c at master · reactos/reactos · GitHub". github.com. Retrieved August 28, 2021.
- ^ "tac(1) - Linux manual page". man7.org.
- ^ "sharkdp/bat". December 2, 2021 – via GitHub.
- ^ UNIX programmers manual (PDF). bitsavers.org (Report). November 3, 1971. p. 32. Archived from the original (PDF) on 2006-06-17.
- ^ GNU Coreutils. "GNU Coreutils manual", GNU, Retrieved on 1 Mars 2017.
- ^ OpenBSD manual page and the GNU Core Utiltites version of cat
- ^ Brian Blackmore (1994-12-05). "Perl or Sed?". Newsgroup: comp.unix.shell. Retrieved 2024-02-12.
- ^ "Merriam Webster's Definition of Abuse". Retrieved 2021-02-25.
- ^ Nguyen, Dan. "Stanford Computational Journalism Lab". stanford.edu. Retrieved 2017-10-08.
External links
- The Single UNIX Specification, Version 4 from The Open Group
: concatenate and print files – Shell and Utilities Reference,
- 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.
- Coreutilsreference
: concatenate and write files –
- OpenBSD General Commands Manual : concatenate and print files –
- FreeBSD General Commands Manual –
- Plan 9 Programmer's Manual, Volume 1 –
- Coreutilsreference : concatenate and write files in reverse –