How to Use grep to Search Text on Linux

What Is grep?

grep (Global Regular Expression Print) searches text for lines matching a pattern and prints them. It's one of the most-used commands on Linux, essential for searching log files, code, config files, and command output.

Basic Usage

grep "error" app.log              # find lines containing "error"
grep "404" /var/log/nginx/access.log
grep "root" /etc/passwd

Most Useful Flags

• -i — case-insensitive search
• -r — recursive search through directories
• -v — invert match (show lines that do NOT match)
• -n — show line numbers
• -c — count matching lines
• -l — list only filenames with matches
• -w — match whole words only
• -A 3 — show 3 lines after the match
• -B 3 — show 3 lines before the match
• -C 3 — show 3 lines before and after (context)

Practical Examples

# Case-insensitive search
grep -i "warning" /var/log/syslog

# Recursive search in all .py files
grep -r "import os" ~/projects/ --include="*.py"

# Show lines that don't contain "success"
grep -v "success" deploy.log

# Count how many times a pattern appears
grep -c "ERROR" app.log

# Show filename and line number
grep -rn "TODO" ~/projects/

# Show context around a match
grep -C 5 "Segmentation fault" /var/log/syslog

Using grep with Pipes

grep shines when combined with other commands using pipes:

ps aux | grep nginx                  # find nginx processes
ls -la | grep "^d"                   # list only directories
cat /etc/passwd | grep "/bin/bash"   # users with bash shell
journalctl -u nginx | grep "error"
history | grep "git commit"          # search command history

Extended Regular Expressions with -E

grep -E "error|warning|fatal" app.log    # match multiple patterns
grep -E "^(GET|POST)" access.log         # lines starting with GET or POST
grep -E "[0-9]{3}" access.log            # three consecutive digits

Search for a Fixed String (No Regex)

grep -F "192.168.1.1" access.log    # treat pattern as literal string

Use -F when your search string contains special regex characters like ., *, or [.