grep - dwilson2547/wiki_demo GitHub Wiki

Here’s a comprehensive grep cheat sheet with common commands and practical examples:

grep (Global Regular Expression Print) is a powerful Linux command for searching text and patterns in files.

• grep Cheat Sheet
• 1. Basic Syntax
• 2. Common Options
• 3. Basic Searches
  • 3.1. Search for a String
  • 3.2. Case-Insensitive Search
  • 3.3. Count Matches
  • 3.4. Show Line Numbers
  • 3.5. Invert Match (Exclude)
  • 3.6. Search in Multiple Files
  • 3.7. Recursive Search in Directories
  • 3.8. Whole Word Match
  • 3.9. Show Context Around Matches
• 4. Advanced Searches with Regex
  • 4.1. Match Lines Starting with a Pattern
  • 4.2. Match Lines Ending with a Pattern
  • 4.3. Match Specific Patterns
  • 4.4. Extended Regex (-E)
  • 4.5. Match Email Addresses
  • 4.6. Match IP Addresses
• 5. Practical Examples
  • 5.1. Find Files Containing a Pattern
  • 5.2. Search in Compressed Files
  • 5.3. Pipe Output to grep
  • 5.4. Search in Command Output
  • 5.5. Highlight Matches
  • 5.6. Search for Empty Lines
  • 5.7. Search for Lines with Only Numbers
• 6. Common Use Cases
• 7. Tips and Tricks

grep [options] "pattern" [file]

grep "pattern" file.txt

Example:

grep "hello" example.txt

• Searches for hello in example.txt.

grep -i "pattern" file.txt

Example:

grep -i "error" logfile.txt

• Matches error, Error, ERROR, etc.

grep -c "pattern" file.txt

Example:

grep -c "404" access.log

• Counts how many times 404 appears in access.log.

grep -n "pattern" file.txt

Example:

grep -n "warning" system.log

• Shows line numbers where warning appears.

grep -v "pattern" file.txt

Example:

grep -v "success" results.txt

• Shows lines that do not contain success.

grep "pattern" file1.txt file2.txt

Example:

grep "TODO" *.py

• Searches for TODO in all .py files.

grep -r "pattern" /path/to/dir/

Example:

grep -r "include" /usr/include/

• Recursively searches for include in /usr/include/.

grep -w "word" file.txt

Example:

grep -w "port" config.txt

• Matches port but not report or export.

grep -A 2 -B 2 "pattern" file.txt

Example:

grep -A 3 -B 1 "error" logfile.txt

• Shows 1 line before and 3 lines after each match.

grep "^pattern" file.txt

Example:

grep "^#" script.sh

• Finds lines starting with # (comments).

grep "pattern$" file.txt

Example:

grep ";$" script.sh

• Finds lines ending with ;.

grep "[0-9]" file.txt       # Lines with numbers
grep "[A-Za-z]" file.txt    # Lines with letters
grep "a.*b" file.txt        # Lines with `a` followed by `b`

Example:

grep "^[A-Z]" names.txt

• Finds lines starting with a capital letter.

grep -E "pattern1|pattern2" file.txt

Example:

grep -E "error|fail|warning" logfile.txt

• Matches error, fail, or warning.

grep -E "[a-zA-Z0-9._-]+@[a-zA-Z0-9._-]+\.[a-zA-Z0-9_-]+" file.txt

Example:

grep -E "\b[A-Za-z0-9._%+-]+@[A-Za-z0-9.-]+\.[A-Za-z]{2,6}\b" contacts.txt

• Extracts email addresses.

grep -E "[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}" file.txt

Example:

grep -E "([0-9]{1,3}\.){3}[0-9]{1,3}" access.log

• Extracts IPv4 addresses.

grep -l "main()" *.c

• Lists all .c files containing main().

zgrep "pattern" file.gz

Example:

zgrep "error" archive.log.gz

• Searches inside a .gz file.

command | grep "pattern"

Example:

dmesg | grep -i "usb"

• Filters dmesg output for USB-related messages.

ps aux | grep "nginx"

• Finds processes related to nginx.

grep --color "pattern" file.txt

Example:

grep --color "TODO" notes.txt

• Highlights TODO in the output.

grep "^$" file.txt

• Finds empty lines.

grep "^[0-9]*$" file.txt

• Matches lines with only numbers.

• Combine with find:

  find /var/log -type f -exec grep -l "error" {} \;

• Use grep with xargs:

  grep -rl "old-text" /path/ | xargs sed -i 's/old-text/new-text/g'

• Save results to a file:

  grep "pattern" file.txt > results.txt

• Search in less: Press / in less to search interactively.