The grep command is one of the most popular and useful commands in Linux. You can use grep to print the lines of text that match a specific pattern. For example, if you want to only display the lines that contain the word green in facts.txt, you can run:
elliot@ubuntu-linux:~$ grep green facts.txt
Grapes are green.
Grass is green.
As you can see, it only printed the two lines that contain the word green.
The grep command can also be very useful when used with pipes. For example, to only list the txt files in your home directory, you can run the command:
elliot@ubuntu-linux:~$ ls | grep txt
all.txt
error.txt
facts2.txt
facts.txt
Mars.txt
mydate.txt
output.txt
planets.txt
soft.txt
You can use the -i option to make your search case-insensitive. For example, if you want to print the lines that contain the word Earth in facts.txt, then use the command:
elliot@ubuntu-linux:~$ grep earth facts.txt
elliot@ubuntu-linux:~$
This will show no result because grep is case-sensitive...