There are many times where a user need to search for
some files, or for strings available in the files. Linux provides a regular
expression tool called ‘GREP’
GREP searches named input files, or the standard input,
and displays lines that match .One or more patterns called regular expressions
or regexes can be used to obtain more better results. GREP can also search
binary files and display records or buffers that contain matches.
Regular Expressions is nothing but a pattern to match for
each input line. A pattern is a sequence of characters. Following all are
examples of pattern:
Basic grep Regular Expression
Grep can be used with any command that generates the
output. Consider the ‘ls’ command combined with grep,
[root@vx111a test]# ls /etc | grep syscon
drwxr-xr-x 11 root root 4096
Aug 3 17:15 sysconfig
I Used the ls command to generate a listing of all files
available and the out put is sent to the grep command for finding a specific
file (syscon in this command)
Now some more examples,
Search For Root in /etc/passwd :
[root@vx111a ~]# grep root /etc/passwd
root:x:0:0:root:/root:/bin/bash
operator:x:11:0:operator:/root:/sbin/nologin
Search For Multiple Strings in a file :
[root@vx111a test]# grep -E 'NTR|naren' test
NTR
naren
OR
Search For Multiple Strings in a File:
[root@vx111a perl]# grep 'mahesh\|NTR' samp
mahesh
NTR
Note : you must use the backslash \ to escape the
OR operator (|).
Search Only Words
Some times when you search for a word ‘sam’ , the grep
searches all the matches including samsam,ramsam . In order to make grep search
for only words ,we can use
grep -w ‘sam” test
Count the Words
[root@vx111a test]# grep -c NTR test
1
[root@vx111a test]# grep -n NTR test
2:NTR ( show the line number too)
List File Names
[root@vx111a test]# grep -i NTR *
test:NTR (shows the File Name )
Results In Color
[root@vx111a test]# grep --color=auto NTR *
test:NTR
One More ,
[root@vx111a test]# grep -E --color=auto 'NTR|Manhesh'
test
Manhesh
NTR
Remove Grep While Grepping Some thing
consider , when you try to grep some thing from the ps
command like this ,it also shows
[root@vx111a test]# ps ux | grep gnome-terminal
root 3349
0.1 0.4 42280 13632 ? Ssl 13:31
0:01 gnome-terminal
root 3740
0.0 0.0 4016
688 pts/1 S+ 13:54 0:00 grep gnome-terminal
Both the search result as well as the grep command is
also shown in the results. In order remove the grep ,we can use
[root@vx111a test]# ps ux | grep -v grep | grep
gnome-terminal
root 3349
0.1 0.4 42280 13632 ? Ssl 13:31
0:01 gnome-terminal
Or
[root@vx111a test]# ps ux | grep [g]nome-terminal
root 3349
0.1 0.4 42280 13632 ? Ssl 13:31
0:01 gnome-terminal
Anchors
There is a facility provided by Grep to search start and
end positions. We can use anchors which help in forcing a regex to match only
at start or end of lines respectively, like
Search For all Lines that start with ‘s’ :
[root@vx111a ~]# grep ^s /etc/passwd
sync:x:5:0:sync:/sbin:/bin/sync
shutdown:x:6:0:shutdown:/sbin:/sbin/shutdown
smmsp:x:51:51::/var/spool/mqueue:/sbin/nologin
sshd:x:74:74:Privilege-separated
SSH:/var/empty/sshd:/sbin/nologin
squid:x:23:23::/var/spool/squid:/sbin/nologin
sabayon:x:86:86:Sabayon user:/home/sabayon:/sbin/nologin
Search for all string that end with ‘bash’
[root@vx111a ~]# grep bash$ /etc/passwd
root:x:0:0:root:/root:/bin/bash
mysql:x:27:27:MySQL Server:/var/lib/mysql:/bin/bash
postgres:x:26:26:PostgreSQL
Server:/var/lib/pgsql:/bin/bash
Match line only containing foo:
grep '^foo$' filename
You can search for blank lines with the following
examples:
grep '^$' filename
Character Class
Using Character Class, we can tell the grep to match
only one out of the several characters. This can be done simply by placing the
characters you want to match between the square brackets.
Search for characters ‘v’ or ‘V’ in the file like
[root@vx111a test]# grep '[vV]' test
Chiranjeevi
Pavan
Bhuvan
Vivek
vivek
mehtavivek
VivekRao
v0091mehta
To
Find Words which starts either with ‘v’ or ‘V’
[root@vx111a test]# grep '[vV]ivek' test
Vivek
vivek
mehtavivek
VivekRao
Some More ,
Some
More Examples
[root@vx111a test]# grep 'v[0]' test
v0091mehta
[root@vx111a test]# grep '[vk'] test
Chiranjeevi
Pavan
Bhuvan
Vivek
vivek
mehtavivek
VivekRao
v0091mehta
Within a bracket expression, the name of a character
class enclosed in "[:" and ":]" stands for the list of all
characters belonging to that class. Standard character class names are:
* [:alnum:] - Alphanumeric characters.
* [:alpha:] - Alphabetic characters
* [:blank:] - Blank characters: space and tab.
* [:digit:] - Digits: '0 1 2 3 4 5 6 7 8 9'.
* [:lower:] - Lower-case letters: 'a b c d e f g h i j k
l m n o p q r s t u v w x y z'.
* [:space:] - Space characters: tab, newline, vertical
tab, form feed, carriage return, and space.
* [:upper:] - Upper-case letters: 'A B C D E F G H I J K
L M N O P Q R S T U V W X Y Z'.
In this example match all upper case letters:
grep '[:upper:]' filename
[root@vx111a perl]# grep [:UPPER:] test
JAGADESH
KIRAN
[root@vx111a perl]# grep '[:alpha:]' test
mehtaVivek
Vevikmehta
Vardham
v0091mehta
pavan
2134lkdsafj
dsankld
Wild
Cards
Grep also provides support for wild card search.
Consider the file ‘sam’ with the contents,
[root@vx111a test]# cat sam
big
bad bug
bag
bigger
boogy
Now if we need to search for words starting with b and
end with g , we can use
[root@vx111a test]# grep b.g sam
big
bad bug
bag
bigger
and also
[root@vx111a test]# grep '\' sam
big
bad bug
bag
Display lines starting with (.) dot
grep ‘^\.[0-9]’ sam
More Advanced Grep options
[root@vx111a test]# cat
sample
100 sai Manager Sales $5,000
200 kai Developer Technology $5,000
300 rai Admin Technology $7,000
400 jai Tester Market $9,000
500 mai Developer Sales $6,000
Grep either Tech Or Sales
[root@vx111a test]# grep
'Tech\|Sales' sample
100 sai Manager Sales $5,000
200 kai Developer Technology $5,000
300 rai Admin Technology $7,000
500 mai Developer Sales $6,000
Using Multiple -e
[root@vx111a test]# grep -e
Tech -e Sales sample
100 sai Manager Sales $5,000
200 kai Developer Technology $5,000
300 rai Admin Technology $7,000
500 mai Developer Sales $6,000
Dev and Tech in One line
[root@vx111a test]# grep -E
'Dev.*Tech' sample
200 kai Developer Technology $5,000
Grep Both Tech and Admin in one line
[root@vx111a test]# grep
Tech sample | grep Admin
300 rai Admin Technology $7,000
Happy Coding , More To Come
