grep Command in Linux/Unix

grep command in Unix is used for pattern or expression searching. It has many OPTIONS that allows to perform various file search operations.

In this article, we will learn common grep command usage with examples:

grep command in Linux/ Unix

Grep full form is “Globally Search For Regular Expression and Print out”.
grep is a pattern or expression searching command.
Prints found matches.
Grep comes with a number of OPTIONS that allows to perform various search operations on files.

SYNTAX:
$grep[options] "pattern to be matched" [filename ]

Options Use
-v Shows all the lines that do not match the searched string
-c Displays only the count of matching lines
-n Shows the matching line and its number
-i Match both (upper and lower) case
-l Shows just the name of the file with the string

Example:

$grep 'hello' file1.txt
Output : searches hello in the file1.txt and outputs/returns the lines containing 'hello'.

Grep command can also be used with meta-characters:

Example:

Input : $grep 'test' *
Output : it searches for test in all the files and directories.
* is a meta-character and returns matching 0 or more preceding characters

-n (–line-number) OPTION with grep command

- grep -n option prints out the matches for the text along with the line numbers.
- Lists line numbers

Example:

$ grep -n 'root' /etc/passwd
Output:
1:root:x:0:0:root:/root:/bin/bash
1042:rootmask:x:0:0:rootmask:/home/rootmask:/bin/csh

The results has line numbers for the text matches.

-c (–count) OPTION with grep command in Unix

grep -c option prints the number of lines of matches

Example:

$grep -c 'test' file1.txt
Output:
2
Note: If there are more than 1 searched word 'test' on line one, option -c would still print 2. 
This is because it is concerned with the number of lines where the matches appear, not the number of matches.

-v (–invert-match) OPTION with grep command in Unix

grep -v option inverts the match, it matches only those lines that do not contain the given word.

Example:

$grep -v 'test' file1.txt
Result:
Hi All
We are learning

The output does not contains the searched pattern.
Note :You can use -n option along with -v t list the line numbers.

-i (–ignore-case) OPTION with grep command in Unix

grep -i option is used to ignore-case sensitivity

Example:

$grep -i 'tEst' file1.txt
No output
$grep -i 'tEst' file1.txt
Output:
In test
Test class

-l (–files-with-matches) OPTION with grep command

grep -l option print file names that match a pattern

Example:

$grep -l test *.txt
Output:
file1.txt

-w (–word-regexp) OPTION with grep command

grep -w option searches for the line containing the exact matching word.
By default, grep matches strings which contain the specified pattern.
This means that grep ‘test’ file1.txt will print the same results as grep ‘est’ file.txt because ‘est’ can be found in test.
With the option -w, grep ensures that the matches are exactly the same pattern as specified.

Example:

$grep -w est file1.txt 
Output: No results

$grep -i -w test file1.txt 
Output:
In test
Test class

-o (–only-matching) OPTION with grep command

grep -o otion prints only the matched pattern
By default, grep prints the line where the matched pattern is found.
With option -o, only the matched pattern is printed line by line.

$grep -i -o yo file1.txt
Output:
test
Test

-r (-recursive) OPTION with grep command

It is used for recursive search.
By default, grep cannot search directories, you will get an error (“Is a directory”).
With option -R, searching files within directories and subdirectories becomes possible.

Example:

$grep -r test *
Output:
myData/file1.txt: In test
myData/file2.txt: Test to learning

for more example refer here

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grep Command in Linux/Unix