Editing Linux 101 hacks (section)

=='''Hack 17. Grep Command'''==


grep command is used to search files for a specific text. This is incredibly powerful command with lot of options.


Syntax: grep <nowiki>[options] </nowiki>pattern <nowiki>[files]</nowiki>


'''How can I find all lines matching a specific keyword on a file?'''


In this example, grep looks for the text John inside /etc/passwd file and displays all the matching lines.


<nowiki># </nowiki>grep John /etc/passwd


jsmith:x:1082:1082:John Smith:/home/jsmith:/bin/bash jdoe:x:1083:1083:John Doe:/home/jdoe:/bin/bash


Option -v, will display all the lines except the match. In the example below, it displays all the records from /etc/password that doesn't match John.


'''Note: '''There are several lines in the /etc/password that doesn’t contain the''' '''word John. Only the first line of the output is shown below.


<nowiki># </nowiki>grep -v John /etc/passwd


jbourne:x:1084:1084:Jason Bourne:/home/jbourne:/bin/bash


'''How many lines matched the text pattern in a particular file?'''


In the example below, it displays the total number of lines that contains the text John in /etc/passwd file.

<nowiki># grep -c John /etc/passwd 2</nowiki>


You can also get the total number of lines that did not match the specific pattern by passing option -cv.


<nowiki># grep -cv John /etc/passwd 39</nowiki>


'''How to search a text by ignoring the case?'''


Pass the option -i (ignore case), which will ignore the case while searching.


<nowiki># </nowiki>grep -i john /etc/passwd


jsmith:x:1082:1082:John Smith:/home/jsmith:/bin/bash jdoe:x:1083:1083:John Doe:/home/jdoe:/bin/bash


'''How do I search all subdirectories for a text matching a specific pattern?'''


Use option -r (recursive) for this purpose. In the example below, it will search for the text "John" by ignoring the case inside all the subdirectories under /home/users.


This will display the output in the format of "filename: line that matching the pattern". You can also pass the option -l, which will display only the name of the file that matches the pattern.


<nowiki># </nowiki>grep -ri john /home/users


/home/users/subdir1/letter.txt:John, Thanks for your contribution.

/home/users/name_list.txt:John Smith

/home/users/name_list.txt:John Doe


<nowiki># </nowiki>grep -ril john /root


/home/users/subdir1/letter.txt /home/users/name_list.txt