Master grep: Powerful Regex Search Techniques for Linux

The grep command in Linux is a powerful text‑search utility that uses regular expressions to locate patterns and print matching lines. Its name stands for Global Regular Expression Print and it is available to all users.

Syntax and Common Options

Basic usage: grep [options] pattern [file...] Key options include: -c: output only the count of matching lines. -i: ignore case distinctions. -h: suppress file name output when searching multiple files. -l: list only the names of files containing matches. -n: show matching lines with their line numbers. -s: suppress error messages for nonexistent or non‑matching files. -v: invert the match, showing lines that do not match. --color=auto: highlight the matching part of the line.

Practical Examples

1) Find lines containing the word png:

[linuxmi@linux:~/linuxmi]$ grep -n 'png' linuxmi.py

2) Find lines that do not contain png:

[linuxmi@linux:~/linuxmi]$ grep -vn 'png' linuxmi.py

3) Search for strings where na is not preceded by v:

[linuxmi@linux:~/linuxmi]$ grep -n '[^v]na' linuxmi.py

4) Search for strings where na is not preceded by any lowercase letter:

[linuxmi@linux:~/linuxmi]$ grep -n '[^a-z]na' linuxmi.py

5) Use ^ to match lines that start with a specific pattern, e.g., lines beginning with ba:

[linuxmi@linux:~/linuxmi]$ grep -n '^ba' linuxmi.py

6) Use [^] to match lines that do not start with a letter:

[linuxmi@linux:~/linuxmi]$ grep -n '^[^a-zA-Z]' linuxmi.py

7) Use $ to match lines ending with a specific character, e.g., lines ending with a period:

[linuxmi@linux:~/linuxmi]$ grep -n '\.$' linuxmi.py

Note: the dot . is a special regex character and must be escaped with a backslash to match a literal period.

8) Use . to match any single character (except newline). Find lines where l and k are separated by exactly two characters:

[linuxmi@linux:~/linuxmi]$ grep -n 'l..k' linuxmi.py

9) Find lines containing consecutive e characters:

[linuxmi@linux:~/linuxmi]$ grep -n 'eee*' linuxmi.py

Here * means “zero or more” repetitions of the preceding character.

10) Find lines that start with l, end with e, and contain at least one x in between:

[linuxmi@linux:~/linuxmi]$ grep -n 'lxx*e' linuxmi.py

11) Find lines that start with l and end with k, with any characters (or none) in between:

[linuxmi@linux:~/linuxmi]$ grep -n 'l.*k' linuxmi.py

12) Use {n} to specify an exact number of repetitions. To match lines containing exactly two e characters:

[linuxmi@linux:~/linuxmi]$ grep -n 'e\{2\}' linuxmi.py

Curly braces are special characters and must be escaped with a backslash.