How to use Grep Command in Linux

The grep (global regular expression print) command is one of the most useful and commonly used command in Linux systems. Grep command searches given input from files and will write each matching line to output. If files are not specified then grep reads from standard input. This guide explains how to use grep command with different options.

Grep Command Syntax#

Below is the basic syntax of grep command.

grep [OPTIONS] PATTERN [FILE…]

Here,

• OPTIONS – Zero or more options. Grep provides a number of options that control its behavior.
• PATTERN – Search pattern.
• FILE – Zero or more input file names.

Ensure that, the user should have read access to the file to search in file.

Use grep Command to Search String in Files#

Mostly, grep command is used to search text in files. For an instance, to search username tecnstuff in /etc/passwd file, type:

grep tecnstuff /etc/passwd

It will show output something like this:

tecnstuff:x:1000:1000:tecnstuff:/home/tecnstuff:/bin/bash

If you want to search words with space then you should enclose using single or double quotes:

grep "Gnome Display Manager" /etc/passwd

Search string in Command output using Grep Command#

If you want to search string from other command’s output and display then you can use pipe the output. In output it will show only matching lines.

For example, you can find the current running processes of your system as www-data user, run below ps command:

ps -ef | grep www-data

www-data   975   973  0 Dec28 ?        00:00:10 nginx: worker process
www-data  1006   904  0 Dec28 ?        00:00:20 php-fpm: pool www
www-data  1007   904  0 Dec28 ?        00:00:19 php-fpm: pool www
tecnstuff 3019  3008  0 04:19 pts/0    00:00:00 grep --color=auto www-data

Recursive Search#

You can use grep command to search recursively for a pattern. Use -r (--recursive) option with grep command. It will search through all files in a specified directory.

For example, search for string tecnstuff.net in all files inside the /etc directory:

grep -r tecnstuff.net /etc

/etc/hosts:127.0.0.1 tecnstuff.net
/etc/nginx/sites-available/tecnstuff.net:    server_name tecnstuff.net   www.tecnstuff.net;

If you would like to search in symbolic links then you can should use -R instead of -r option:

grep -R tecnstuff.net /etc

It will display the matching lines with path:

/etc/hosts:127.0.0.1 tecnstuff.net
/etc/nginx/sites-available/tecnstuff.net:    server_name tecnstuff.net   www.tecnstuff.net;
/etc/nginx/sites-enabled/tecnstuff.net:    server_name tecnstuff.net   www.tecnstuff.net;

Use grep to search exact words only#

When search any word by default grep will return all line wherever keyword is used. For example, if you search for sweta user then grep will match with sweta123, test_sweta and more. You can get exact match using -w option along with grep command. It will force grep to match only whole word sweta:

grep -w "sweta" file

Show Only the Filename#

When need to print only file name where search pattern match. Use the -l ( or --files-with-matches) option with grep command:

grep -l tecnstuff.net *.conf

Above command will search in all files ending with .conf in current working directory and will print only the file names which contains the string tecnstuff.net.

tmux.conf
proxy.conf

Force grep invert match#

To print the invert of match you can use -v option with grep command. It will match only those line which doesn’t contains the given word.

For example print all line that do not contain the word gnu:

grep -v gnu /path/to/file

Case Insensitive Search#

By default, the grep command is case sensitive. So the uppercase and lowercase characters are considered as different. If you want to ignore case when searching, use -i (or --ignore-case) option.

For example, /home/tecnstuff/test_file file have words nginx, nGinx, NginX and run below command:

grep Unix /home/tecnstuff/test_file

Above command will not match and doesn’t show anything in output.

But if you execute grep command with -i option then it will match to the both uppercase and lowercase and return output.

grep -i Unix /home/tecnstuff/test_file

It should show output as below:

nginx
nGinx
NginX

Show Line Numbers#

It’s very convenience to check in file by line number. So if you want to print line number along with the output then you can use -n option:

grep -n gzip /etc/nginx/nginx.conf

By running above command it will print below output.

48:     gzip on;
49:     gzip_disable "msie6";
51:     gzip_vary on;
52:     gzip_proxied any;
53:     gzip_comp_level 6;
54:     gzip_buffers 16 8k;
55:     gzip_http_version 1.1;

Count Matches#

If you required only count of line matches, use -c (--count) option with grep. It will return number of counts. Below command will return the number of match with gzip string:

grep -c gzip /etc/nginx/nginx.conf

It will print number 8 as output.

8

Conclusion#

The grep command is regularly used command and you can search for a specific pattern inside of files. You can get more information about Grep at Grep Manual page.

If you have any questions or suggestion, feel free to leave a comment below.