awk

Short for Aho, Weinberger, and Kernighan; a programming language in itself, used for text processing using regular expressions. This is described further in “How awk-ward.”

grep

Short for Global [search for] Regular Expressions and Print; prints lines matching an input pattern (optionally with a specified number of lines of leading and/or trailing context). The grep command can take input from standard input or from files.

Common variants include agrep (“approximate grep” from the Univ. of AZ), fgrep, and egrep.

head

Prints the first few lines from a file (or standard input). The number of lines can be specified with the -n flag.

perl

A programming language whose scripts can be easily embedded in shell scripts using the -e flag. Perl's regular expression language is somewhat richer than basic regular expressions (and easier to read than character classes in extended regular expressions), making it popular for text processing use.

sed

Short for stream editor; performs more complex text substitutions using regular expressions.

sort

Sorts a series of lines. By default, sort reads these lines from its standard input. After its standard input is closed, it sorts them and prints the results to its standard output.

tail

Prints the last few lines from of a file (or standard input). The number of lines can be specified with the -n flag. Alternatively, you can specify the starting position as a byte or line offset from either the start or end of the file.

tee

Copies standard input to standard output, saving a copy into a file (or multiple files).

tr

Replaces one character with another.

uniq

Filters out adjacent lines that match.