grep Command in Linux
The grep command in Linux is a powerful command-line utility used for searching text files or input streams for lines that match a specific pattern.
What is grep Command in Linux?
grep searches through text - usually files or command outputs looking for lines that match a given pattern. It then prints the matching lines to the terminal.
Basic Syntax of grep Command :
grep [options] pattern [file...]
Example of grep Command in Linux
1. Search for a Word in a File
grep "error" logfile.txt
🔹 Finds all lines in logfile.txt that contain the word "error".
2. Case-Insensitive Search
grep -i "error" logfile.txt
🔹 Matches "Error", "ERROR", "error", etc.
3. Search Recursively in Directories
grep -r "main()" .
🔹 Searches the current directory and subdirectories for the pattern "main()".
4. Count the Number of Matches
grep -c "failed" logfile.txt
🔹 Returns the number of lines that contain "failed".
5. Display Line Numbers
grep -n "timeout" config.txt
🔹 Shows the line numbers where "timeout" occurs.
Useful Options in grep Command
| Option | Description |
|---|---|
| -i | Case-insensitive search |
| -r | Recursive search through directories |
| -c | Count matching lines |
| -n | Show line numbers |
| -l | List only filenames with matches |
| -w | Match whole words |
| -A | Show A lines after a match |
| -B | Show B lines before a match |
| -C | Show C lines of context (before & after) |
Example : Showing Context Around a Match
grep -C 2 "error" logfile.txt
🔹 Displays 2 lines before and after each matching line.
Advanced grep command usage : Regular Expressions
grep supports regular expressions, allowing for more flexible pattern matching.
Example: Match lines that start with "Failed"
grep "^Failed" file.txt
Example: Match lines ending in ".log"
grep "\.log$" file.txt
Combining grep with Other Commands
grep becomes even more powerful when combined in pipelines:
Find errors in real-time logs:
tail -f syslog.log | grep "ERROR"
Search for users with /bin/bash in /etc/passwd:
cat /etc/passwd | grep "/bin/bash"
Related Commands
- egrep: Extended grep (now deprecated, use grep -E)
- fgrep: Fixed-string grep (use grep -F)
- zgrep: For searching in compressed .gz files
grep is very useful in pattern matching and text filtering on the command line as whether you're troubleshooting logs, analyzing data, or scripting automation tasks , understanding grep will help you.