grep command

The grep command allows you to search one file or multiple files for lines that contain a pattern. Exit status is 0 if matches were found, 1 if no matches were found, and 2 if errors occurred.

Syntax

The syntax for the grep command is:

grep [options] pattern [files]

Options

Example-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,

Example: Consider the below file as an input.

$cat > geekfile.txt

unix is great os. unix is opensource. unix is free os.
learn operating system.
Unix linux which one you choose.
uNix is easy to learn.unix is a multiuser os.Learn unix .unix is a powerful.

1. Case insensitive search : The -i option enables to search for a string case insensitively in the give file. It matches the words like “UNIX”, “Unix”, “unix”.

$grep -i "UNix" geekfile.txt

Output:

unix is great os. unix is opensource. unix is free os.
Unix linux which one you choose.
uNix is easy to learn.unix is a multiuser os.Learn unix .unix is a powerful.

2. Displaying the count of number of matches : We can find the number of lines that matches the given string/pattern

$grep -c "unix" geekfile.txt

Output:

2

3. Display the file names that matches the pattern : We can just display the files that contains the given string/pattern.

$grep -l "unix" *

or
 
$grep -l "unix" f1.txt f2.txt f3.xt f4.txt

Output:

geekfile.txt

4. Checking for the whole words in a file : By default, grep matches the given string/pattern even if it found as a substring in a file. The -w option to grep makes it match only the whole words.

$ grep -w "unix" geekfile.txt

Output:

unix is great os. unix is opensource. unix is free os.
uNix is easy to learn.unix is a multiuser os.Learn unix .unix is a powerful.

5. Displaying only the matched pattern : By default, grep displays the entire line which has the matched string. We can make the grep to display only the matched string by using the -o option.

$ grep -o "unix" geekfile.txt

Output:

unix
unix
unix
unix
unix
unix

6. Show line number while displaying the output using grep -n : To show the line number of file with the line matched.

$ grep -n "unix" geekfile.txt

Output:

1:unix is great os. unix is opensource. unix is free os.
4:uNix is easy to learn.unix is a multiuser os.Learn unix .unix is a powerful.

7. Inverting the pattern match : You can display the lines that are not matched with the specified search sting pattern using the -v option.

$ grep -v "unix" geekfile.txt

Output:

learn operating system.
Unix linux which one you choose.

8. Matching the lines that start with a string : The ^ regular expression pattern specifies the start of a line. This can be used in grep to match the lines which start with the given string or pattern.

$ grep "^unix" geekfile.txt

Output:

unix is great os. unix is opensource. unix is free os.

9. Matching the lines that end with a string : The $ regular expression pattern specifies the end of a line. This can be used in grep to match the lines which end with the given string or pattern.

$ grep "os$" geekfile.txt

10.Specifies expression with -e option. Can use multiple times :

$grep –e "Agarwal" –e "Aggarwal" –e "Agrawal" geekfile.txt

11. -f file option Takes patterns from file, one per line.

$cat pattern.txt

Agarwal
Aggarwal
Agrawal

$grep –f pattern.txt  geekfile.txt