The grep command stands for Global Regular Expression Print. It is one of the most commonly used commands in Linux for searching text patterns inside files or output from other commands. It helps users find lines that contain a specific keyword, phrase, or regular expression.

What does grep do?

grep scans through one or more files and prints all lines that match a given pattern. It is useful for:

• Searching log files

• Finding specific configuration details

• Filtering command outputs

• Matching patterns using regular expressions

Basic Syntax

grep [options] "pattern" filename

Common Examples

1. Search for a word in a file

grep "error" logfile.txt

Shows all lines containing the word "error".

2. Case-insensitive search

grep -i "error" logfile.txt

Matches error, Error, ERROR, etc.

3. Recursive search in directories

grep -r "config" /etc/

Searches for the word "config" in all files under /etc.

4. Show line numbers of matches

grep -n "root" /etc/passwd

Displays the matching lines with line numbers.

5. Count matching lines

grep -c "failed" auth.log

Shows how many lines contain the word "failed".

6. Show only the matched word

grep -o "admin" users.txt

Prints only the matched word.

7. Match whole words only

grep -w "cat" animals.txt

Matches "cat" but not "category".

Useful Options

Using grep with Pipes

ps aux | grep "apache"

Finds processes related to "apache".