The grep command is used to search text or searches the given file for lines containing a match to the given strings or words. By default, grep displays the matching lines. Use grep to search for lines of text that match one or many regular expressions, and outputs only the matching lines. grep is considered as one of the most useful commands on Unix and other Linux operating systems.
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The name, "grep", derives from the command used to perform a similar operation, using the Unix/Linux text editor ed:
The grep command syntax
The syntax is as follows:
grep 'word' filename grep 'word' file1 file2 file3 grep 'string1 string2' filename cat otherfile | grep 'something' command | grep 'something' command option1 | grep 'data' grep --color 'data' fileName
How do I use grep command to search a file?
Search /etc/passwd file for boo user, enter:
$ grep boo /etc/passwd
You can force grep to ignore word case i.e match boo, Boo, BOO and all other combination with the -i option:
$ grep -i "boo" /etc/passwd
Use grep recursively
You can search recursively i.e. read all files under each directory for a string "192.168.1.5"
$ grep -r "192.168.1.5" /etc/
$ grep -R "192.168.1.5" /etc/
/etc/ppp/options:# ms-wins 192.168.1.50 /etc/ppp/options:# ms-wins 192.168.1.51 /etc/NetworkManager/system-connections/Wired connection 1:addresses1=192.168.1.5;24;192.168.1.2;
You will see result for 192.168.1.5 on a separate line preceded by the name of the file (such as /etc/ppp/options) in which it was found. The inclusion of the file names in the output data can be suppressed by using the -h option as follows:
$ grep -h -R "192.168.1.5" /etc/
$ grep -hR "192.168.1.5" /etc/
# ms-wins 192.168.1.50 # ms-wins 192.168.1.51 addresses1=192.168.1.5;24;192.168.1.2;
Use grep to search words only
When you search for boo, grep will match fooboo, boo123, barfoo35 and more. You can force the grep command to select only those lines containing matches that form whole words i.e. match only boo word:
$ grep -w "boo" file
Use grep to search 2 different words
Use the egrep command as follows:
$ egrep -w 'word1|word2' /path/to/file
Count line when words has been matched
The grep can report the number of times that the pattern has been matched for each file using -c (count) option:
$ grep -c 'word' /path/to/file
Pass the -n option to precede each line of output with the number of the line in the text file from which it was obtained:
$ grep -n 'root' /etc/passwd
1:root:x:0:0:root:/root:/bin/bash 1042:rootdoor:x:0:0:rootdoor:/home/rootdoor:/bin/csh 3319:initrootapp:x:0:0:initrootapp:/home/initroot:/bin/ksh
Grep invert match
You can use -v option to print inverts the match; that is, it matches only those lines that do not contain the given word. For example print all line that do not contain the word bar:
$ grep -v bar /path/to/file
UNIX / Linux pipes and grep command
grep command often used with shell pipes. In this example, show the name of the hard disk devices:
# dmesg | egrep '(s|h)d[a-z]'
Display cpu model name:
# cat /proc/cpuinfo | grep -i 'Model'
However, above command can be also used as follows without shell pipe:
# grep -i 'Model' /proc/cpuinfo
model : 30 model name : Intel(R) Core(TM) i7 CPU Q 820 @ 1.73GHz model : 30 model name : Intel(R) Core(TM) i7 CPU Q 820 @ 1.73GHz
How do I list just the names of matching files?
Use the -l option to list file name whose contents mention main():
$ grep -l 'main' *.c
Finally, you can force grep to display output in colors, enter:
$ grep --color vivek /etc/passwd
If you enjoyed the grep tutorial, then you might like to read our "Regular Expressions in Grep" tutorial.