12 useful examples of Linux grep command

 

grep

In this article, we are going to provide you with useful examples of Linux grep command including the two variant programs: egrep and fgrep. grep (global regular expression print) is used to search for text strings and regular expressions line by line which match a specified pattern within one or more files.

Listed below are some simple and useful examples of how to use the grep command on Linux and search for a string in one/multiple files.

Search for lines containing ‘DB_USER’ in WordPress configuration file (wp-config.php):

# grep 'DB_USER' wp-config.php
define('DB_USER', 'wpuser');

Search for all PHP files containing ‘str_replace’ text inside the ‘wp-admin’ directory of a WordPress installation:

# # grep "str_replace" admin*.php
admin-ajax.php: add_action( 'wp_ajax_' . $_GET['action'], 'wp_ajax_' . str_replace( '-', '_', $_GET['action'] ), 1 );
admin-ajax.php: add_action( 'wp_ajax_' . $_POST['action'], 'wp_ajax_' . str_replace( '-', '_', $_POST['action'] ), 1 );
admin-header.php:$admin_body_class .= ' branch-' . str_replace( array( '.', ',' ), '-', floatval( $wp_version ) );
admin-header.php:$admin_body_class .= ' version-' . str_replace( '.', '-', preg_replace( '/^([.0-9]+).*/', '$1', $wp_version ) );
admin-header.php:$admin_body_class .= ' locale-' . sanitize_html_class( strtolower( str_replace( '_', '-', get_locale() ) ) );

If you want to add line numbers to search results, use the following command:

# grep -n "str_replace" admin*.php
admin-ajax.php:73:      add_action( 'wp_ajax_' . $_GET['action'], 'wp_ajax_' . str_replace( '-', '_', $_GET['action'] ), 1 );
admin-ajax.php:76:      add_action( 'wp_ajax_' . $_POST['action'], 'wp_ajax_' . str_replace( '-', '_', $_POST['action'] ), 1 );
admin-header.php:157:$admin_body_class .= ' branch-' . str_replace( array( '.', ',' ), '-', floatval( $wp_version ) );
admin-header.php:158:$admin_body_class .= ' version-' . str_replace( '.', '-', preg_replace( '/^([.0-9]+).*/', '$1', $wp_version ) );
admin-header.php:160:$admin_body_class .= ' locale-' . sanitize_html_class( strtolower( str_replace( '_', '-', get_locale() ) ) );

Search for all PHP files whose contents mention ‘str_replace’ text which has either capital or small letters in it, located in the ‘wp-admin’ directory recursively (i.e. read all files under each directory) and list the file names only:

# grep -ril "str_replace" admin*.php
admin-ajax.php
admin-header.php

grep command is often used in a Unix/Linux pipeline with other commands. Listed below are some more advanced examples of how to use the grep command on Linux:

Search for lines containing ‘eval’ in all PHP files located in the current directory:

#find . -name '*.php' -exec grep -l 'eval' {} ;
./wp-admin/includes/image.php
./wp-admin/includes/class-wp-upgrader.php
./wp-admin/includes/class-pclzip.php
./wp-admin/includes/media.php
./wp-admin/includes/update-core.php
./wp-admin/includes/ajax-actions.php
./wp-admin/includes/class-wp-automatic-updater.php
./wp-admin/includes/class-wp-posts-list-table.php
./wp-admin/user-edit.php
./wp-admin/edit-tag-form.php

List all Apache processes:

# ps aux | grep http
root      1259  0.0  0.0 114644   984 pts/0    S+   01:37   0:00 grep --color=auto http
root      1471  0.0  2.3 502008 24508 ?        Ss   Jun27   0:03 /usr/sbin/httpd -k start
nobody   31256  0.0  3.8 510140 40412 ?        S    00:00   0:01 /usr/sbin/httpd -k start
nobody   31257  0.0  4.2 510220 44404 ?        S    00:00   0:02 /usr/sbin/httpd -k start
nobody   31258  0.0  4.5 511156 47924 ?        S    00:00   0:02 /usr/sbin/httpd -k start
nobody   31259  0.0  5.4 514240 56648 ?        S    00:00   0:02 /usr/sbin/httpd -k start
nobody   31260  0.0  3.9 510388 41668 ?        S    00:00   0:01 /usr/sbin/httpd -k start
nobody   31585  0.0  4.2 511156 44188 ?        S    00:13   0:01 /usr/sbin/httpd -k start

See which sockets belong to process ID 31585:

# lsof -p 31585 | grep -Ei 'cwd|unix|sock'
httpd   31585 nobody  cwd    DIR    144,142     4096   35120509 /
httpd   31585 nobody  mem    REG       8,18            38427466 /opt/cpanel/ea-php56/root/usr/lib64/php/modules/sockets.so (path dev=144,142)
httpd   31585 nobody  mem    REG       8,18            36064050 /usr/lib64/apache2/modules/mod_unixd.so (path dev=144,142)

Display total number of Apache connections on port 80:

# netstat -an | grep :80 |wc -l
1627

Remove all frozen messages in Exim mail queue immediately:

#exim -bpr | grep frozen | awk {'print $3'} | xargs exim -Mrm
Message 1dPhFh-0000t6-D0 has been removed
Message 1dPenR-0000Ls-S8 has been removed
Message 1dPexx-0000OD-A6 has been removed

Find files containing a text pattern (e.g. hello):

find . -iname "*.txt" -exec grep -l "hello" {} +

To search for multiple patterns at one time, you can simply use the egrep command. egrep is the same as grep -E.

#egrep 'cachedir|exclude' /etc/yum.conf
cachedir=/var/cache/yum/$basearch/$releasever
exclude=lm_sensors*

The fgrep searches a file or list of files for a fixed pattern string. fgrep is the same as grep –F.

#fgrep 'cachedir' /etc/yum.conf
cachedir=/var/cache/yum/$basearch/$releasever

 

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