cat (Unix)

Example of some cat options:[9]

• -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

The Jargon File version 4.4.7 lists this as the definition of cat:

1. To spew an entire file to the screen or some other output sink without pause (syn. blast).
2. 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 standard output than to concatenate two or more files. The name cat for the former operation is just as unintuitive as, say, LISP's cdr.

Useless use of cat (UUOC) is common Unix jargon for command line constructs that only provide a function of convenience to the user.[10] In computing, the word "abuse",[11] 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 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 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.[12] 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