How to use Linux Grep Command to Find Strings

Linux and UNIX-like systems, all system configuration information is stored and manipulated in plain text form. The grep tool allows to search of text, such as a file or output of the command, for a term or pattern matching regular expressions. Grep is used to search text for patterns specified by the user. It is one of the most useful and powerful commands on Linux and UNIX like operating system.

When grep finds match in a line, it copies in to the screen that is stdout. Using grep command you can replace the string in given file. Grep provides a number of additional options that, if specified , force the program to output the context for each match.

Syntax
grep [ option(s) ] pattern [file(s) ]

Grep Command
The basic usage of grep command is to search for a specific string in the specified file as shown below.



[root@localhost ~]# grep root /etc/passwd
root:x:0:0:root:/root:/bin/bash
operator:x:11:0:operator:/root:/sbin/nologin
Above example grep searches the /etc/passwd file for root string and redirected to stdout.

Options

-i searches for the given string/pattern case-insensitively. So it matches all the words such as “hal”, “HAL” case insensitively as shown below.

Even you can use Line and, anchors, character classes, wild cards,

[root@localhost ~]# grep -i HAL /etc/passwd
halt:x:7:0:halt:/sbin:/sbin/halt
haldaemon:x:68:68:HAL daemon:/:/sbin/nologin
Searching string in a given file
Use below grep command to search specific string in a file

[root@localhost ~]# grep "root" /etc/passwd
root:x:0:0:root:/root:/bin/bash
operator:x:11:0:operator:/root:/sbin/nologin
Searching string in multiple files
Use below grep command to search specific string from multiple files

[root@localhost ~]# grep "root" /etc/passwd /etc/group
/etc/passwd:root:x:0:0:root:/root:/bin/bash
/etc/passwd:operator:x:11:0:operator:/root:/sbin/nologin
/etc/group:root:x:0:root
/etc/group:bin:x:1:root,bin,daemon
/etc/group:daemon:x:2:root,bin,daemon
/etc/group:sys:x:3:root,bin,adm
/etc/group:adm:x:4:root,adm,daemon
/etc/group:disk:x:6:root
/etc/group:wheel:x:10:root
Regular Expressions in files
? The preceding item is optional and matched at most once.
• * The preceding item will be matched zero or more times.
• + The preceding item will be matched one or more times.
• {n} The preceding item is matched exactly n times.
• {n,} The preceding item is matched n or more times.
• {,m} The preceding item is matched at most m times.
• {n,m} The preceding item is matched at least n times, but not more than m times.

[root@localhost ~]# grep " *oot" /etc/passwd
root:x:0:0:root:/root:/bin/bash
operator:x:11:0:operator:/root:/sbin/nologin
Searching full word using grep –w
The -w flag for grep will make the given expression match only whole words.

[root@localhost ~]# grep -iw "samba" /etc/samba/smb.conf
# This is the main Samba configuration file. You should read the
# here. Samba has a huge number of configurable options (perhaps too
# For a step to step guide on installing, configuring and using samba,
# read the Samba-HOWTO-Collection. This may be obtained from:
Grep with option -A
-A option which displays the N lines after the string match.

[root@localhost ~]# grep -A 2 "root" /etc/passwd
root:x:0:0:root:/root:/bin/bash
bin:x:1:1:bin:/bin:/sbin/nologin
daemon:x:2:2:daemon:/sbin:/sbin/nologin
operator:x:11:0:operator:/root:/sbin/nologin
games:x:12:100:games:/usr/games:/sbin/nologin
gopher:x:13:30:gopher:/var/gopher:/sbin/nologin

-B option which displays the N lines before the string match
When the string matches it will display N lines before the string for a given file

[root@localhost ~]# grep -B 2 "root" /etc/passwd
root:x:0:0:root:/root:/bin/bash
--
news:x:9:13:news:/etc/news:
uucp:x:10:14:uucp:/var/spool/uucp:/sbin/nologin
operator:x:11:0:operator:/root:/sbin/nologin
-E option can be used search multiple strings
-e is strictly the flag for indicating the pattern you want to match against. -E controls whether you need to escape certain special characters

[root@localhost ~]# ls | grep -E “li|ovo”
linux24
ovo8_linux24.sh
[root@localhost ~]#
Or we can use

[root@localhost ~]# ls | grep ‘li\|ovo’
linux24
ovo8_linux24.sh
[root@localhost ~]#
Exclude specific file from searching
Search string in all files except syslog.log

[root@localhost ~]#grep -r string * | grep -v ‘/\syslog.log/’