grep – Practical Search on Linux Shell

The grep command searches text for patterns. It’s a powerful tool for quickly finding information inside files or command outputs.

Basic Syntax

grep [OPTIONS] PATTERN [FILE...]

• PATTERN: Text or regular expression to search for.
• FILE: One or more files to search in. If none, reads from stdin.

Options

Practical Everyday Examples

1. Search for a word in a single file

grep "error" /var/log/syslog

Find all lines containing “error” in the system log.

2. Case-insensitive search

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

Find “warning”, “Warning”, or “WARNING”.

3. Search recursively in a directory

grep -r "TODO" ~/projects/

Find all TODO comments in your project files.

4. Show line numbers with matches

grep -n "fail" /var/log/auth.log

Quickly jump to exact lines where “fail” appears.

5. Count how many times a word appears in a file

grep -c "sshd" /var/log/auth.log

6. Show only filenames with matches

grep -l "main()" *.c

List C files containing main().

7. Invert match — show lines without the pattern

grep -v "DEBUG" app.log

Filter out all debug messages from logs.

8. Search command output (use pipe |)

dmesg | grep -i "usb"

Find USB-related kernel messages.

9. Search multiple patterns (OR) with -E (extended regex)

grep -E "error|fail|warning" /var/log/syslog

10. Use grep to filter process list

ps aux | grep apache

11. Search whole words only (-w)

grep -w "cat" file.txt

Avoid matching words like “catalog”.

12. Use context options: show lines before/after matches

grep -C 2 "error" app.log

Show 2 lines before and after each match for better context.

13. Search hidden files/directories recursively

grep -r --exclude-dir=".git" "fixme" .

Search everywhere except .git.

14. Save grep output to a file

grep "fail" /var/log/syslog > fails.txt

15. Colorize output for better visibility (if not default)

grep --color=always "pattern" file.txt

Combining grep with Other Tools

Filter and count unique IPs in Apache logs

grep "GET" access.log | awk '{print $1}' | sort | uniq -c | sort -nr

Find processes running more than 100 MB RAM

ps aux | grep apache | awk '$6 > 100000'

Find TODOs and fixme comments in code (recursive)

grep -riE "todo|fixme" ./src/

Tips & Tricks

• Use grep -r with --exclude or --exclude-dir to skip files or directories.
• Combine with head or tail to view only first or last matches.
• Use xargs to apply commands on grep results, e.g.:

grep -rl "TODO" . | xargs wc -l

More Resources

man grep
info grep

Feel free to ask if you want examples for regex, scripting use cases, or log monitoring with grep!