The grep command is one of the most powerful commands in Linux. It is used for searching a pattern inside files. The pattern can range from a simple alphabet to a complex regular expression. There are various scenarios where grep can be useful. I will try to list some of them in this post.
List all the lines in a file that contain the word
grep -i 'hello world' main.txt
The -i option is used to ignore case distinctions i.e makes the search case insensitive. The above command will try to find the lines in the file main.txt that contain the word hello world.
We may give more than 1 file if we need.
grep -i 'hello world' main.txt main.c
List just the name of the matching files
grep -l 'main' *.py
This will list all Python modules (*.py files) inside the current directory that contain the word main inside them.
List the name of the matching files alongside the matching lines
grep -H 'main' *.py
Search recursively inside directories
grep -r 'hello' /home
Both -r and -R specify the search to be recursive, except the fact that -R also follows symlinks.
Search recursively only through files that match a particular pattern
grep -ir 'main' include='*.cpp' /home
This will look recursively but only inside the C++ files(end with .cpp)
Search only for a word
Our last search would even match with word mains or really_main but if we want it to just match a word not just a part of the word, we can use the option -w
grep -irw 'main' include='*.cpp' /home
For more control, you may use the \<, \> to match the start and end of words. For example
grep '\<main' *
will search only for the words starting with main. So this will match the word mains but not really_mains.
List the file names with no matching lines
grep -L 'import' *.py
Invert the match
grep -v 'import' *.py
Count the number of lines when the word matches
grep -c 'import' *.py
Print the line number alongside the matched line
grep -n 'import' *.py
Print the context around matching lines
grep -C 2 'math.log' *.py
This will print 2 lines of context around each matching line.
Using Regular Expression
grep -e '[[:digit:]]' info.txt
The -e option is used to specify regular expression patterns. In this case, the pattern is a digit. You can use more complex expressions according to your needs.
Using pipes to redirect output from another command
tail /var/log/apache2/error.log | grep -e '[[:digit:]]'
These were some of the use cases that I thought could be useful for you when learning about the grep command. It can help you in saving time, especially when searching through a chunk of files. I hope it helps you swim to deeper depths into the world of Linux.
Till we meet in another post, keep hacking and take care.