Working with `grep`

grep stands for Global Regular Expression Print. It searches for patterns (text strings or regex) in files or input streams and prints matching lines.

Basic Syntax


grep [options] pattern [file...]

Common Usage Examples

Search in a file


grep "error" /var/log/syslog

Finds lines containing "error" in the file.

Search recursively in directories


grep -r "TODO" /home/user/projects/

Searches for "TODO" in all files under the given directory.

Case-insensitive search


grep -i "warning" logfile.txt

Matches warning, Warning, or WARNING.

Count matching lines


grep -c "fail" result.log

Returns how many lines contain "fail".

Display line numbers


grep -n "404" access.log

Adds line numbers before matching lines.

Use regular expressions


grep "^start" file.txt

Matches lines starting with “start”.


grep "end$" file.txt

Matches lines ending with “end”.

Invert match (show non-matching lines)


grep -v "success" log.txt

Displays lines that do not contain "success".

Search in output of a command (stream)


dmesg | grep usb

Searches for “usb” in the kernel ring buffer output.

Highlight matches

Most modern systems highlight matches by default, but you can force it:


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

Important Options Summary

Option	Description
`-i`	Case-insensitive
`-v`	Invert match
`-r` or `-R`	Recursive search in directories
`-n`	Show line numbers
`-c`	Count matching lines
`-l`	Show only filenames with matches
`-w`	Match whole words only
`-e`	Use multiple patterns
`--color`	Highlight matching text

Piping with `grep`

You can use grep with pipes to search command output:


ps aux | grep apache

Finds Apache-related processes.


journalctl -xe | grep sshd

Filters SSH-related logs.

Summary

grep is essential for searching text patterns in files or output.
Works with exact strings or regex.
Combine with pipes for powerful log and process filtering.
Use options to customize search behavior (case sensitivity, line numbers, etc.).

Working with grep