Use the Grep command

Use the Grep command
GuideAll linux systems provide a search tool named grep (global regular expression print, global regular expression output. The grep command is very useful when pattern-based searches are performed on the content of one or more files. The mode can be a single character, multiple characters, a single word, or a sentence. When the command matches the mode specified when the command is executed, grep outputs a line in the include mode, but does not modify the content of the original file.Example 1 search mode (Word) in the file)

Find the word "linuxtechi" in the/etc/passwd file"

root@Linux-world:~# grep linuxtechi /etc/passwdlinuxtechi:x:1000:1000:linuxtechi,,,:/home/linuxtechi:/bin/bashroot@Linux-world:~#

Example 2 search for multiple files

root@Linux-world:~# grep linuxtechi /etc/passwd /etc/shadow /etc/gshadow    /etc/passwd:linuxtechi:x:1000:1000:linuxtechi,,,:/home/linuxtechi:/bin/bash    /etc/shadow:linuxtechi:$6$DdgXjxlM$4flz4JRvefvKp0DG6re:16550:0:99999:7:::/etc/gshadow:adm:*::syslog,linuxtechi    /etc/gshadow:cdrom:*::linuxtechi    /etc/gshadow:sudo:*::linuxtechi    /etc/gshadow:dip:*::linuxtechi    /etc/gshadow:plugdev:*::linuxtechi    /etc/gshadow:lpadmin:!::linuxtechi    /etc/gshadow:linuxtechi:!::    /etc/gshadow:sambashare:!::linuxtechiroot@Linux-world:~#

Example 3 use the-L parameter to list the file names that contain the specified mode

root@Linux-world:~# grep -l linuxtechi /etc/passwd /etc/shadow /etc/fstab /etc/mtab    /etc/passwd    /etc/shadowroot@Linux-world:~#

Example 4 use the-N parameter to search for the specified mode in the file and display the row number that matches the row

root@Linux-world:~# grep -n linuxtechi /etc/passwd    39:linuxtechi:x:1000:1000:linuxtechi,,,:/home/linuxtechi:/bin/bashroot@Linux-world:~#

Example 5 use the-V parameter to output rows that do not contain the specified mode

Output all lines without the word "linuxtechi" in the/etc/passwd file

root@Linux-world:~# grep -v linuxtechi /etc/passwd

Example 6 use the ^ symbol to output all rows starting with a specified mode

The Bash script treats the ^ symbol as a special character to specify the beginning of a line or word. For example, output all rows starting with "root" in the/etc/passes File

root@Linux-world:~# grep ^root /etc/passwd    root:x:0:0:root:/root:/bin/bashroot@Linux-world:~#

Example 7 use the $ symbol to output all rows ending in the specified mode

Output all rows ending with bash in the/etc/passwd file

root@Linux-world:~# grepbash$ /etc/passwd    root:x:0:0:root:/root:/bin/bash    linuxtechi:x:1000:1000:linuxtechi,,,:/home/linuxtechi:/bin/bashroot@Linux-world:~#

The Bash script treats the dollar ($) symbol as a special character to specify the end of a row or word.

Example 8 use the-R parameter to recursively find a specific mode

root@Linux-world:~# grep -r linuxtechi /etc/    /etc/subuid:linuxtechi:100000:65536    /etc/group:adm:x:4:syslog,linuxtechi    /etc/group:cdrom:x:24:linuxtechi    /etc/group:sudo:x:27:linuxtechi    /etc/group:dip:x:30:linuxtechi    /etc/group:plugdev:x:46:linuxtechi    /etc/group:lpadmin:x:115:linuxtechi    /etc/group:linuxtechi:x:1000:    /etc/group:sambashare:x:131:linuxtechi    /etc/passwd-:linuxtechi:x:1000:1000:linuxtechi,,,:/home/linuxtechi:/bin/bash    /etc/passwd:linuxtechi:x:1000:1000:linuxtechi,,,:/home/linuxtechi:/bin/bash............................................................................

The above command recursively searches for the "linuxtechi" word in the/etc directory.

Example 9 use Grep to find all empty rows in the file

root@Linux-world:~# grep ^$ /etc/shadowroot@Linux-world:~#

No blank lines exist in the/etc/shadow file, so there is no output

Example 10 use the-I parameter Search Mode

The-I parameter of the grep command ignores the case sensitivity of characters during search. Let's look at an example to find the word "LinuxTechi" in the paswd file.

nextstep4it@localhost:~$ grep -i LinuxTechi /etc/passwd    linuxtechi:x:1001:1001::/home/linuxtechi:/bin/bashnextstep4it@localhost:~$

Example 11 use the-E parameter to find multiple modes

For example, if you want to find the 'linuxtechi' and 'root' words in a grep command and use the-e parameter, we can find multiple modes.

root@Linux-world:~# grep -e "linuxtechi" -e "root" /etc/passwd    root:x:0:0:root:/root:/bin/bash    linuxtechi:x:1000:1000:linuxtechi,,,:/home/linuxtechi:/bin/bashroot@Linux-world:~#

Example 12 use-F to specify the mode to be searched with a file

First, create a search mode file "grep_pattern" in the current directory.

root@Linux-world:~# cat grep_pattern    ^linuxtechi    root    false$root@Linux-world:~#

Now, try to search with the grep_pattern File

root@Linux-world:~# grep -f grep_pattern /etc/passwd

Example 13 use the-C parameter to calculate the number of matching Modes

In the above example, we use the-c command in the grep command to calculate the number of matched modes.

root@Linux-world:~# grep -c -f grep_pattern /etc/passwd    22root@Linux-world:~#

Example 14 output match the first or last N rows of the specified mode row

A) use the-B parameter to output the first four rows of matching rows.

root@Linux-world:~# grep -B 4"games" /etc/passwd

B) use the-A parameter to output the last four rows of matching rows.

root@Linux-world:~# grep -A 4"games" /etc/passwd

C) use the-C parameter to output four rows before and after matching rows

root@Linux-world:~# grep -C 4"games" /etc/passwd

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