grep command

grep - print lines that match patterns

grep searches for PATTERNS in each FILE or input. PATTERNS is one or more patterns separated by newline characters, and grep prints each line that matches a pattern. Typically PATTERNS should be quoted when grep is used in a shell command. It’s great for finding specific lines in logs, code, or any text data.

Usage: grep [OPTION]... PATTERNS [FILE]...

• OPTION: Flags which enhances the grep abilities.
• FILE: A FILE or standard input to search. If no FILE is given, recursive searches examine the working directory, and nonrecursive searches read standard input.
• PATTERNS: The text or regular expression to search for. PATTERNS is one or more patterns separated by newline characters.

Examples

• Basic Search in a File

  Search for a string in a file and display matching lines.

  $ grep "error" logfile.txt
  • Prints all lines in logfile.txt containing "error".
  • Case-sensitive by default.

• Case-Insensitive Search

  Use -i to ignore case differences.

  $ grep -i "Error" logfile.txt
  • Matches "error", "ERROR", "Error", etc.

• Searching Multiple Files

  Pass multiple files to search them all.

  $ grep "warning" file1.txt file2.txt
  • Shows matches from both files, prefixed with filenames (e.g., file1.txt:warning here).

• Recursive Search

  Use -r to search all files in a directory recursively.

  grep -r "TODO" /home/user/code/
  • Finds "TODO" in all files under /home/user/code/.

• Line Numbers

  Add -n to show line numbers with matches.

  $ grep -n "error" logfile.txt
  • Output: 5: disk error occurred (line 5 contains "error").

• Matching Whole Words

  Use -w to match only complete words.

  $ grep -w "run" script.sh
  • Matches "run" but not "running" or "runner".

• Inverted Search

  Use -v to show lines that don’t match the pattern.

  $ grep -v "success" logfile.txt
  • Displays all lines without "success".

• Counting Matches

  Use -c to count matching lines per file.

  grep -c "error" logfile.txt
  • Output: 3 (if "error" appears on 3 lines).

• Piping Input

  Search output from another command.

  ps aux | grep "firefox"
  • Finds processes containing "firefox" in their details.

• Basic Regular Expressions

  grep supports regex for advanced patterns:

  • . : Any single character
  • * : Zero or more occurrences
  • ^ : Start of line
  • $ : End of line

  grep "^ERROR" logfile.txt
  • Matches lines starting with "ERROR".

  Example with Wildcard:

  grep "err.*" logfile.txt
  • Matches "error", "erratic", etc.

To get help related to the grep command use --help option

$ grep --help