grep

What it does

grep searches input — files or stdin — for lines matching a pattern and prints the matching lines. The name comes from the ed editor command g/re/p ("global regular expression print"). It's one of the most-used tools for searching logs, source code, and configuration files from the command line.

Beginner examples

• grep "error" app.log — print lines containing "error"
• grep -i "error" app.log — case-insensitive match
• grep -n "TODO" main.py — show line numbers
• grep -v "debug" app.log — print lines that do NOT match

grep -in "warning" /var/log/syslog

Advanced examples

• Search recursively through a directory: grep -r "TODO" ./src
• Use extended regex for alternation: grep -E "error|fatal" app.log
• Show 3 lines of context around each match: grep -C 3 "exception" app.log
• Count matches instead of printing lines: grep -c "error" app.log
• Search multiple files and show only filenames with matches: grep -l "TODO" src/*.py

grep -rl "console.log" ./src

Common mistakes

• Forgetting that grep uses basic regular expressions by default, so +, ?, and | need escaping (or use -E/egrep).
• Searching case-sensitively by accident and getting zero results — sanity-check with -i when a search seems to come up empty.
• Running grep pattern * in a directory with subdirectories and missing matches inside them, because the shell glob doesn't recurse — use -r instead.
• Piping ls into grep to filter filenames instead of using find or shell globbing.

Tips

• Use grep --color=auto to highlight matches in the terminal.
• Combine -o with a regex to extract just the matching text instead of the whole line.
• grep -q suppresses output and only sets an exit code — handy in shell scripts (if grep -q pattern file; then ... fi).

Best practices

• Anchor patterns with ^/$ when you mean "starts with" or "ends with" to avoid unintended partial matches.
• Prefer grep -F (fixed string) over regex when searching for a literal string with special characters — faster and avoids escaping bugs.
• In scripts, always check grep's exit code rather than assuming success; it returns non-zero when no lines match.

Real-world use cases

• Searching application logs for error patterns during an incident.
• Auditing a codebase for leftover TODO/FIXME comments before a release.
• Filtering ps aux output to find a specific running process: ps aux | grep nginx.

Common interview questions

• What's the difference between grep, egrep, and fgrep? egrep is grep -E (extended regex); fgrep is grep -F (literal string matching, no regex).
• How do you search recursively while ignoring binary files? grep -rI pattern . — -I skips binary files.
• How would you find lines that do NOT match a pattern? grep -v pattern file.