How to search for strings inside files in a folder?

I assume that your first question is about a GUI alternative to the grep command. I can't help you with that, I always find the command line very effective.

As for the command line, try

grep "test" /var/x/*

If you want to search recursively (i.e. not only in /var/x/, but also in subdirectories thereof), do

grep -R "test" /var/x/

To avoid grepping the files which grep thinks to be binary, use the -I option:

grep -I "test" /var/x/

If grep thinks a file is binary (based on first few bytes of the file), it will assume it does not match instead of going through the whole file.

You can use searchmonkey. The tool is available in the repositories, so you can simply

sudo apt-get install searchmonkey

On the other hand, command line search with grep is really intended for that...

Here is a screenshot from searchmonkey

You can use regexxer it is a great GUI search/replace tool for regular expressions.

you can download by:

sudo apt-get install regexxer

Try Recoll, best GUI one I ever used. To install recoll in all currently supported versions of Ubuntu open the terminal and type:

sudo apt install recoll

It needs some time to index the files first (you can define blacklist path or extensions or mime).

You can use the grep command from terminal:

grep -r string *

This command will find all occurrences of "string" in all the files under the current directory (or subdrectories).

For hidden files, you can use:

grep -r string ..