grep

Explanation

The grep command searches for patterns in each file. Patterns can be simple strings or complex regular expressions. It prints each line that matches a pattern. grep is an essential tool for parsing logs, finding specific code, or filtering output from other commands.

Basic Syntax

grep [OPTION]... PATTERNS [FILE]...

Common Flags and Options

Flag	Description
-i, --ignore-case	Ignore case distinctions in patterns and input data.
-v, --invert-match	Select non-matching lines.
-r, -R, --recursive	Read all files under each directory, recursively.
-n, --line-number	Prefix each line of output with the 1-based line number.
-l, --files-with-matches	Print only names of FILEs containing matches.
-c, --count	Print only a count of selected lines per FILE.

Real-world Examples

1. Search for a string in a file:

   grep "error" /var/log/syslog

2. Search case-insensitively for a pattern:

   grep -i "warning" mylog.txt

3. Search for a pattern recursively in a directory:

   grep -r "TODO" src/