What is the difference between grep, egrep and fgrep?

They are the same program with different default regex modes. grep uses basic regular expressions, egrep is equivalent to grep -E for extended regex, and fgrep is grep -F for fixed literal strings. The egrep and fgrep names are deprecated in favour of the flags.

How do I search recursively through a directory?

Use -r (or -R to follow all symlinks) with a starting directory. Combine with --include or --exclude glob patterns to limit which files are searched, for example grep -r --include='*.js' TODO src.

How do I show lines around a match?

Use context flags: -A NUM prints lines after the match, -B NUM prints lines before, and -C NUM prints both. They are invaluable for understanding a match in its surrounding code or log context.

What does grep -P add over grep -E?

-P enables Perl-compatible regular expressions (PCRE), which support features missing from POSIX extended regex such as lookahead and lookbehind, \d and \w shorthands, and non-greedy quantifiers. It is a GNU extension and not available everywhere.

How do I use grep just to test if a pattern exists?

Use -q for quiet mode: grep produces no output and exits with status 0 on the first match, or non-zero if none is found. This makes it ideal inside an if condition, often combined with -F for a literal string.

grep Flags Reference

grep flags

grep searches text for lines matching a pattern and prints them. Its behaviour is shaped almost entirely by flags: whether matching is case-sensitive, what counts as a match, how results are reported, how much context is shown, which files are searched recursively, and which regular-expression dialect applies. This page is a searchable, offline reference to every common grep flag, with a note on whether each is POSIX-standard or a GNU extension.

How it works

grep reads input line by line and tests each line against the pattern. Flags fall into clear groups:

Matching control changes what matches: -i ignores case, -v inverts, -w and -x require whole-word or whole-line matches, and -F treats the pattern as a literal string instead of a regex.
Output control changes what is printed: -c counts, -l/-L list file names, -o prints only the matched text, -n adds line numbers, and -q prints nothing but sets the exit status.
Context flags -A, -B, -C print surrounding lines.
File selection flags -r/-R recurse, while --include, --exclude and --exclude-dir filter which paths are searched.
Regex flavor is chosen with -G (basic, default), -E (extended) or -P (Perl/PCRE).

The exit status is itself useful: 0 means at least one match, 1 means no match, and 2 means an error.

Tips and examples

Find all TODO comments under a source tree, but only in JavaScript files:

grep -rn --include='*.js' 'TODO' src/

Count how many lines mention an error, case-insensitively:

grep -ic 'error' app.log

Show three lines of context around each match to read it in situ:

grep -C3 'panic' server.log

Use grep purely as a condition without printing anything:

if grep -qF "feature_x" config.ini; then
  echo "feature enabled"
fi

Prefer -F whenever your search text is a fixed string with regex metacharacters like dots or brackets — it is both faster and avoids surprises.

grep Flags Reference

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grep flags

How it works

Tips and examples