grep

grep [options] pattern [file...]

Option	Description
grep pattern file	Displays lines matching the pattern.
-i	Performs a case-insensitive search.
-r	Searches directories recursively.
-v	Displays lines that do not match the pattern (inverted search).
-n	Displays line numbers for matching lines.
-l	Displays only the names of files with matching lines.
-L	Displays only the names of files with no matching lines.
-c	Displays only the count of matching lines.
-E	Uses extended regular expressions (ERE), equivalent to egrep.
-F	Treats the pattern as a fixed string (does not interpret regular expressions).
-w	Displays only lines where the pattern matches a whole word.
-x	Displays only lines where the pattern matches the entire line.
-A N	Also displays N lines after each matching line (After).
-B N	Also displays N lines before each matching line (Before).
-C N	Also displays N lines before and after each matching line (Context).
--include="*.php"	Limits the files searched by extension during recursive search.
--exclude-dir=.git	Excludes the specified directory from the search.

The following file is used in these examples.

app.log

2026-03-05 09:58:10 INFO  server started
2026-03-05 10:00:01 ERROR database connection failed
2026-03-05 10:02:15 INFO  retrying connection
2026-03-05 10:03:30 WARNING disk usage 85%
2026-03-05 10:05:22 ERROR timeout after 30s
2026-03-05 10:06:00 INFO  connection restored

Search for lines containing ERROR in a file (case-sensitive).

grep "ERROR" app.log
2026-03-05 10:00:01 ERROR database connection failed
2026-03-05 10:05:22 ERROR timeout after 30s

Use -i to search without case sensitivity.

grep -i "error" app.log
2026-03-05 10:00:01 ERROR database connection failed
2026-03-05 10:05:22 ERROR timeout after 30s

Use -n to display results with line numbers.

grep -n "ERROR" app.log
2:2026-03-05 10:00:01 ERROR database connection failed
5:2026-03-05 10:05:22 ERROR timeout after 30s

Use -c to display only the count of matching lines.

grep -c "ERROR" app.log
2

Use -v to display lines that do not match the pattern (for example, to exclude comment lines).

grep -v "^#" config.txt
host=localhost
port=3306
debug=true

Use -E with extended regular expressions to search for multiple keywords.

grep -E "ERROR|WARNING" app.log
2026-03-05 10:00:01 ERROR database connection failed
2026-03-05 10:03:30 WARNING disk usage 85%
2026-03-05 10:05:22 ERROR timeout after 30s

Use -C N to display N lines before and after each matching line.

grep -C 1 "WARNING" app.log
2026-03-05 10:02:15 INFO  retrying connection
2026-03-05 10:03:30 WARNING disk usage 85%
2026-03-05 10:05:22 ERROR timeout after 30s

Use -r to search recursively through a directory. Add -l to display only filenames.

grep -rl "TODO" ./src
./src/index.php
./src/utils.php

Use --include to filter files by extension and --exclude-dir to exclude specific directories.

grep -r --include="*.php" --exclude-dir=.git "TODO" .
./src/index.php:// TODO: add validation
./src/utils.php:// TODO: add error handling

$ grep -r --include="*.php" pattern . is a commonly used pattern for finding where functions and variables are used in PHP projects. Today, faster alternatives such as ripgrep (rg) and ag (The Silver Searcher) are also widely available.

For text substitution, see sed. For field processing, see awk.