Using grep command in Linux

What is grep ?

Grep stands for Global Regular Expression Print. Grep utility is usually used to find lines in a file/files containing particular strings and substrings.

Grep comes preinstalled on any modern Linux distribution, so we can start using it without having to install anything.

Before we start looking at usage examples of grep utility, let’s create two files in our working directory (I named them inventory.txt and stock.txt )with the following content:

stock.txt:

ID	Brand	Type	S/N
10	Lenovo	Laptop	198756
11	Asus	PC	665321
12	Acer	PC	333777
13	Samsung	Laptop	223588
14	Dell	PC	125854
15	Dell	Laptop	982354

inventory.txt:

ID	Brand	Type	S/N
1	IBM	Laptop	123456
2	Apple	PC	654321
3	HP	PC	333777
4	Sony	Laptop	225588
5	Toshiba PC	123654
6	Toshiba Laptop	987654

Search for a string in a file using grep.

The most basic usage of grep is the following grep Toshiba inventory.txt

After the grep commands itself we specify the string we want find (Toshiba) and then the filename (inventory.txt).

orkhans@matrix:~/grep$ grep Toshiba inventory.txt 
5	Toshiba PC	123654
6	Toshiba Laptop	987654

Search for a string in multiple files using grep.

If we search for a string in multiple files we can specify several files like this:

grep Laptop inventory.txt stock.txt

This grep command returns the lines containing “Laptop” string from both files and specifies the name of the file for each line:

orkhans@matrix:~/grep$ grep Laptop inventory.txt stock.txt
inventory.txt:1	IBM	Laptop	123456
inventory.txt:4	Sony	Laptop	225588
inventory.txt:6	Toshiba Laptop	987654
stock.txt:10	Lenovo	Laptop	198756
stock.txt:13	Samsung	Laptop	223588
stock.txt:15	Dell	Laptop	982354

We can also use wildcard character * if we want to perform a search in all files in the current directory.

Look at the output of command grep Laptop *:

orkhans@matrix:~/grep$ grep Laptop *
inventory.txt:1	IBM	Laptop	123456
inventory.txt:4	Sony	Laptop	225588
inventory.txt:6	Toshiba Laptop	987654
stock.txt:10	Lenovo	Laptop	198756
stock.txt:13	Samsung	Laptop	223588
stock.txt:15	Dell	Laptop	982354

Make grep command ignore case

grep command is case sensitive by default, therefore the  command grep laptop * does not return anything:

orkhans@matrix:~/grep$ grep laptop *
orkhans@matrix:~/grep$

We can make grep ignore case by using -i switch: grep -i laptop *

orkhans@matrix:~/grep$ grep -i laptop *
inventory.txt:1	IBM	Laptop	123456
inventory.txt:4	Sony	Laptop	225588
inventory.txt:6	Toshiba Laptop	987654
stock.txt:10	Lenovo	Laptop	198756
stock.txt:13	Samsung	Laptop	223588
stock.txt:15	Dell	Laptop	982354

Inverse search using grep

Sometimes we need to find lines in a file NOT containing a particular string. We can do it using grep, because it has a switch -v to do just that.

Let’s find all lines not containing “Laptop” string in our files:

orkhans@matrix:~/grep$ grep -v Laptop *
inventory.txt:ID	Brand	Type	S/N
inventory.txt:2	Apple	PC	654321
inventory.txt:3	HP	PC	333777
inventory.txt:5	Toshiba PC	123654
stock.txt:ID	Brand	Type	S/N
stock.txt:11	Asus	PC	665321
stock.txt:12	Acer	PC	333777
stock.txt:14	Dell	PC	125854

Find a number of lines containing a string

We can ask grep to find the number of lines containing the required string for each file. Switch -c instructs grep to do that.

The command is grep -c Laptop *

orkhans@matrix:~/grep$ grep -c Laptop *
inventory.txt:3
stock.txt:3

Find lines coming before or after the lines with matching strings

It’s possible to make grep print out lines going before or after the matching lines.

The switch -A (please note that it is capital A) instructs grep to print the lines after the matching line. The number after -A switch tells how many lines we want to print.

This command will output the matching line with the two following lines:

orkhans@matrix:~/grep$ grep -A 2 Acer *
stock.txt:12	Acer	PC	333777
stock.txt-13	Samsung	Laptop	223588
stock.txt-14	Dell	PC	125854

The switch -B (please note that it is capital B) instructs grep to print the lines before the matching line. The number after -B switch tells how many lines we want to print:

orkhans@matrix:~/grep$ grep -B 2 Sony *
inventory.txt-2	Apple	PC	654321
inventory.txt-3	HP	PC	333777
inventory.txt:4	Sony	Laptop	225588

Search for a regex pattern using grep

We can use grep to search for a regex pattern rather than plain string. We should provide regex pattern in double quotes.

orkhans@matrix:~/grep$ grep "A.*3" *
inventory.txt:2	Apple	PC	654321
stock.txt:11	Asus	PC	665321
stock.txt:12	Acer	PC	333777

Conclusion

These were some basic usage examples of grep. Grep is a very useful and flexible utility, especially because of regex support. Regex allows us to build very complex patterns, which helps a lot when dealing with large text files. You can learn more about regex here.

Thank you for reading.