Power of grep command in Linux.

 

The grep filter searches a file for a particular pattern of characters, and displays all lines that contain that pattern. The pattern that is searched in the file is referred to as the regular expression (grep stands for global search for regular expression and print out).

Syntax:

grep [options] pattern [files]

Options:

-c : This prints only a count of the lines that match a pattern
-h : Display the matched lines, but do not display the filenames.
-i : Ignores, case for matching
-l : Displays list of a filenames only.
-n : Display the matched lines and their line numbers.
-v : This prints out all the lines that do not matches the pattern
-e exp : Specifies expression with this option. Can use multiple times.
-f file : Takes patterns from file, one per line.
-E : Treats pattern as an extended regular expression (ERE)
-w : Match whole word
-o : Print only the matched parts of a matching line,
 with each such part on a separate output line.

-A n : Prints searched line and nlines after the result.
-B n : Prints searched line and n line before the result.
-C n : Prints searched line and n lines after before the result.


Grep commands:

grep command which stands for "global regular expression print" processes text line by line and prints any lines which match a specified pattern.

grep --version: gives the version of the grep package.

grep --help: this will give you all the options that can be used with grep command.

man grep: gives the manual of the grep command.

grep cat file1: gives the lines that match the word cat.

grep spathan /etc/passwd: gives the line which matches spathan.

cat /etc/group | grep spathan: gives the line which matches spathan

grep -c spathan /etc/passwd: gives the number of lines which matches spathan

grep -i spathan /etc/passwd: ignore lowercase and uppercase grep -n spathan /etc/passwd: gives the line numbers which match with spathan

grep -v spathan /etc/passwd: display everything except the line containing spathan.

grep -v spathan /etc/passwd | awk '{print $1}': display only 1st column of the lines which does not contain spathan word Is-1| grep -i desktop: displays the line which contains desktop directory.
......................... 
egrep: when you want to search for 2 words in the output.
 egrep -i "spathan l agowda" /etc/passwd: print all the lines which contains both words spathan and agowda.