Cat (Unix)
cat is a standard Unix utility that reads files sequentially, writing them to standard output. The name is derived from its function to concatenate files.History
cat was part of the early versions of Unix, e.g., Version 1, and replaced pr, a PDP-7 utility for copying a single file to the screen.The version of
cat bundled in GNU coreutils was written by Torbjorn Granlund and Richard Stallman.Usage
The Single Unix Specification defines the operation ofcat 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.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
Example of somecat options:- , number non-blank output lines
- implies but also display end-of-line characters as
- , number all output lines
- , squeeze multiple adjacent blank lines
- implies, but also display tabs as
- use unbuffered I/O for stdout. POSIX does not specify the behavior without this option.
- , 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.
cat does not provide a way to concatenate Unicode text files that have a Byte Order Mark 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 formats, 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 and DV formats, which are fundamentally simple streams of packets.Examples
| 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 |
| command | cat | Cancel "command" special behavior when it writes directly to TTY |
Unix culture
Jargon file definition
The Jargon File version 4.4.7 lists this as the definition ofcat:Useless use of cat
Useless use of cat is common Unix jargon for command line constructs that only provide a function of convenience to the user. This is also referred to as "cat abuse". The activity of fixing instances of UUOC is sometimes called demoggification. Example of a commoncat 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 :
command arg1 arg2 argn < filename
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. Also, one wrong use of the redirection symbol ">" instead of "<" 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
out
with:
cat in | command | command2 > out
tac
tac is a Linux command that allows viewing files line-by-line, beginning from the last line. It is named by analogy withcat.Usage:
Usage: tac ... ...
Write each FILE to standard output, last line first.
With no FILE, or when FILE is -, read standard input.
Mandatory arguments to long options are mandatory for short options too.
-b, --before attach the separator before instead of after
-r, --regex interpret the separator as a regular expression
-s, --separator=STRING use STRING as the separator instead of newline
--help display this help and exit
--version output version information and exit