Grep(global regular expression print) command is used in Linux to find and search using regular expressions throughout the Linux system.
syntax: grep 'string' filename
or
syntax: filename grep 'string'
eg -> grep -i 'warning' logfile.txt
To search recursively in all the directories -r option
grep 'config' -r /etc/nginx
In the above command, we're finding a config file inside the /etc/nginx directory. By using -r, it will find all the sub-directories inside the nginx directory
To count the lines where strings are matched with -c option
grep -c 'INFO' logfile.txt
This command will display the number of lines in which the INFO text is present in the logfile.txt
To display the line number where the search pattern contains with -n option
grep -n 'WARNING' logfile.txt
To ignore case sensitivity with the -i option
grep -i 'info' logfile.txt
To invert the output of a file with the -v flag
grep -vi 'info' logfile.txt
This will print everything on the console that doesn't contain info
Redirecting the results from other commands to grep by using pipe |
dpkg -l | grep 'package-name'
eg -
dpkg -l | grep "chrome"
here dpkg command will list the install packages inside the Linux system and with the help of pipe we check that the Chrome browser is installed in the system
grep with REGEX
A REGEX or REgular EXpression is a sequence of characters that is used to match pattern.
eg ->
^ Matches characters at the beginning of a line
$ Matches characters at the end of a line
"." Matches any character
[a-z] Matches any characters between A and Z
[^ ..] Matches anything apart from what is contained in the brackets
To print certain characters from a file
syntax - grep ^character filename
To display the lines begin with the number "0" in our logfile.txt file
grep ^0 logfile.txt
To display the lines end with character "p"
grep p$ logfile.txt
Colorizing grep results with --color flag
If you're working on a system that doesn't display the search pattern in a different color then use --color flag.
grep --color 'INFO' logfile.txt
For more grep options
If you need to learn more about grep command usage, then use --help will display the usage of all flags and options that can be used with the grep command.
grep --help