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Difference between grep and fgrep command

Last Updated : 11 Jul, 2025

This is the powerful grep and fgrep commands used to search text in a Linux-based system. Despite their similarity in use, which is for matching patterns in file strings, they still appear to be different in their own approach and capabilities. Grep supports regular expressions, including flexible and complex patterns of searching, whereas fgrep, short for "fixed grep," focuses on the search for strings without an interpretation of regular expressions.

The grep filter searches a file for a particular pattern of characters and displays all lines that contain that pattern. The fgrep filter searches for fixed-character strings in a file or files.

Here are the basic syntaxes for grep and fgrep:

grep [options] pattern [files]

fgrep [options] pattern [files]

Feature

'grep'

'fgrep' (Deprecated, now grep -F)

Pattern Type

Supports regular expressions for pattern matching.

Searches for fixed strings only, without interpreting patterns.

Special Characters

Interprets special characters (like *, .) as part of the regex.

Treats all characters literally, including special characters.

Performance

Slightly slower when processing complex regular expressions.

Faster for exact string matching, as it skips regex parsing.

Command Usage

Example: grep "error" file.txt

Example: fgrep "error" file.txt or grep -F "error" file.txt

Use Case

Used for flexible, pattern-based searches.

Used for simple, exact string searches.

Recursive Search

Supports recursive search with -r option.

Does not directly support recursive search (use grep -F -r).

Deprecated

No, still widely used.

Yes, fgrep is deprecated; use grep -F instead.

Options

Supports various options like -i, -v, -n, etc.

Supports similar options like -i, -v, -n, but no regex support.

The main difference between both commands is:

• String matching algorithm used by them.
• fgrep always uses the Aho-Corasick algorithm that worst O(m+n) complexity.
• grep command always uses the modified version of Commentz-Walter algorithm which has worst case O(mn) complexity.
• fgrep command interprets the PATTERN as a list of fixed strings separated by newlines. But grep always interpreted as regular expressions.

Consider the below file named as para2

Hi, are you using geeksforgeeks for learning computer science concepts.
Geeksforgeeks is best for learning.

Consider the below Words:

are
using
geeksforgeeks
learning
concepts

$grep -f word para

Output:

Hi, are you using geeksforgeeks for learning computer science concepts.
Geeksforgeeks is best for learning.

$fgrep -f word para

Output:

Hi, are you using geeksforgeeks for learning computer science concepts.
Geeksforgeeks is best for learning.

Consider the below File :

Hi, @re you usin.g geeks*forgeeks for learni\ng computer science con/cepts.
Geeks*forgeeks is best for learni\ng.

Consider the below Words :

@re
usin.g
geeks*forgeeks
learni\ng
con/cepts

grep -f word para

Output:

Hi, @re you usin.g geeks*forgeeks for learni\ng computer science con/cepts.

fgrep -f word para

Output:

Hi, @re you usin.g geeks*forgeeks for learni\ng computer science con/cepts.
Geeks*forgeeks is best for learni\ng.

Option

Description

-i

Ignore case while matching patterns.

-v

Invert the match; show lines that do not match the pattern.

-r

Recursively search files in directories (for grep, but not supported in fgrep).

-n

Show line numbers with output.

-l

List file names where matches occur instead of displaying the actual lines.

1. -i: This option ignores case sensitivity.

grep -i 'error' /var/log/syslog
fgrep -i 'error' /var/log/syslog

2. -v: This option inverts the match, displaying lines that do not match the pattern.

grep -v 'error' /var/log/syslog
fgrep -v 'error' /var/log/syslog

3. -n: Displays the line number along with each matching line.

grep -n 'error' /var/log/syslog
fgrep -n 'error' /var/log/syslog

4. -l: This option lists the files that contain the pattern but does not show the actual lines.

grep -l 'error' *.log
fgrep -l 'error' *.log

In conclusion, both grep and fgrep commands may be used while doing file searches, though they do differ in functionality, the grep command supports complex regular expressions. It is, therefore highly useful for flexible pattern matching. fgrep is optimized for fixed-character strings where the search could, most probably, be performed significantly faster if no regular expressions were required.

If you need to use pattern-matching capabilities that are greater than the basic capability, then grep is a better choice. But if you're doing an exact string match, the speed would improve for fgrep since it wouldn't have to process regex.

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