grep

grep is a command-line utility for searching text using patterns. It reads input from files or standard input and prints lines that match the given pattern. The name originates from the Unix command g/re/p in the editor ed, and represents “global regular expression print.” It was developed for Unix and became a core tool in many Unix-like environments, including implementations in GNU coreutils and other systems.

Usage and behavior: grep [options] pattern [file...]. By default it uses basic regular expressions to match lines and prints those lines to standard output. For extended regular expressions, many implementations offer -E (or use the egrep variant). For fixed strings, -F (or fgrep) can be used. Common options include -i for case-insensitive matching, -v to invert matches, -n to show line numbers, -c to count matches, -l to list matching files, -r or -R to search directories recursively, -w to match whole words, -x to match whole lines, and --color=auto to highlight matches. Some versions provide -P to use Perl-compatible regular expressions.

Variants and scope: grep has related forms such as fgrep (fixed strings), egrep (extended regex), and, in many systems, a Perl-compatible option (-P). It is frequently used as a filter in pipelines, often combined with tools like sort, uniq, or sed.

Examples: grep 'pattern' file.txt; grep -i 'error' /var/log/*.log; grep -R --color=auto 'TODO' /project. grep remains a fundamental tool for text processing, scripting, and log analysis.