LINUX GREP

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The grep command is used to search text. It searches the given file for lines containing a match to the given strings or words. It is one of the most useful commands on Linux and Unix-like system. Let us see how to use grep on a Linux or Unix like system.

Did you know?

The name, “grep”, derives from the command used to perform a similar operation, using the Unix/Linux text editor ed:
g/re/p

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 to search a file?

Search /etc/passwd file for boo user, enter:
$ grep boo /etc/passwd
Sample outputs:

foo:x:1000:1000:foo,,,:/home/foo:/bin/ksh

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

How to 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/
OR
$ grep -R "192.168.1.5" /etc/
Sample outputs:

/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/
OR
$ grep -hR "192.168.1.5" /etc/
Sample outputs:

# ms-wins 192.168.1.50
# ms-wins 192.168.1.51
addresses1=192.168.1.5;24;192.168.1.2;

How to 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

How to use grep to search 2 different words

Use the egrep command as follows:
$ egrep -w 'word1|word2' /path/to/file

How can I 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
Sample outputs:

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

Force 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
Sample outputs:

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
Sample outputs:

Grep command in action on Linux and Unix like system
Grep command in action