107-the-grep-command.md

The grep command

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

Examples:

1. To search the contents of the destination.txt file for a string("KeY") case insensitively.

grep -i "KeY" destination.txt

2. Displaying the count of number of matches

grep -c "key" destination.txt

3. We can search multiple files and only display the files that contains the given string/pattern.

grep -l "key" destination1.txt destination2.txt destination3.xt destination4.txt

4. To show the line number of file with the line matched.

grep -n "key" destination.txt

5. If you want to grep the monitored log files, you can add the --line-buffered to search them in real time.

tail -f destination.txt | grep --line-buffered "key"

Syntax:

The general syntax for the grep command is as follows:

grep [options] pattern [files]

Additional Flags and their Functionalities:

Short Flag	Long Flag	Description
-c	--count	print a count of matching lines for each input file
-h	--no-filename	Display the matched lines, but do not display the filenames
-i	--ignore-case	Ignores, case for matching
-l	--files-with-matches	Displays list of a filenames only.
-n	--line-number	Display the matched lines and their line numbers.
-v	--invert-match	This prints out all the lines that do not matches the pattern
-e	--regexp=	Specifies expression with this option. Can use multiple times
-f	--file=	Takes patterns from file, one per line.
-F	--fixed-strings=	Interpret patterns as fixed strings, not regular expressions.
-E	--extended-regexp	Treats pattern as an extended regular expression (ERE)
-w	--word-regexp	Match whole word
-o	--only-matching	Print only the matched parts of a matching line, with each such part on a separate output line.
	--line-buffered	Force output to be line buffered.