grep Basics

grep is a powerful command-line utility used for searching text patterns in files or standard input. Below, we cover the basics of grep, including its syntax, options, and practical examples.

Basic Syntax

The basic syntax for grep is:

grep [options] pattern [file...]

• pattern: The text or regular expression to search for.

• file: The file(s) to search.

If no file is specified, grep reads from standard input.

Commonly Used Options

• -i: Ignore case distinctions in the pattern.

• -v: Invert the match to select non-matching lines.

• -r or -R: Recursively search files in a directory.

• -l: Print only the names of files with matches.

• -c: Count the number of matching lines.

• -n: Prefix each line of output with its line number.

• -H: Print the filename for each match (useful when searching multiple files).

• -E: Interpret the pattern as an extended regular expression (ERE).

• --color: Highlight matching strings in the output.

Examples

Basic Search

Search for lines containing the word "error":

grep "error" file.txt

Ignore Case

Search for "error" regardless of case:

grep -i "error" file.txt

Invert Match

Display lines that do not contain "error":

grep -v "error" file.txt

Recursive Search

Search for "TODO" in all files in the current directory and subdirectories:

grep -r "TODO" .

Show Line Numbers

Print matching lines with their line numbers:

grep -n "error" file.txt

Search Multiple Files

Search for "error" in multiple files:

grep "error" file1.txt file2.txt

Count Matches

Count the number of lines containing "error":

grep -c "error" file.txt

Show Only Filenames

List filenames containing matches:

grep -l "error" *.log

Use Regular Expressions

Search for lines that start with "ERROR":

grep "^ERROR" file.txt

Search for lines ending with ".log":

grep "\.log$" file.txt

Highlight Matches

Highlight the matching text:

grep --color "error" file.txt

Combine Options

Search recursively, ignore case, and show line numbers:

grep -rin "error" .

Advanced Examples

Search for Whole Words

Match the whole word "error" (not substrings like "errors"):

grep -w "error" file.txt

Search with Extended Regular Expressions

Use extended regex to search for "error" or "warning":

grep -E "error|warning" file.txt

Search Specific File Types

Search for "TODO" only in .c files:

grep -r --include="*.c" "TODO" .

Exclude Specific Files

Search for "TODO" but exclude .log files:

grep -r --exclude="*.log" "TODO" .

Summary

grep is an essential tool for text processing, offering versatile options for searching patterns efficiently. By combining its features with regular expressions and options, you can perform complex searches and analyze data effectively.