How to check if a file contains a specific string using Bash

if grep -q SomeString "$File"; then
  Some Actions # SomeString was found
fi

You don't need [[ ]] here. Just run the command directly. Add -q option when you don't need the string displayed when it was found.

The grep command returns 0 or 1 in the exit code depending on the result of search. 0 if something was found; 1 otherwise.

$ echo hello | grep hi ; echo $?
1
$ echo hello | grep he ; echo $?
hello
0
$ echo hello | grep -q he ; echo $?
0

You can specify commands as an condition of if. If the command returns 0 in its exitcode that means that the condition is true; otherwise false.

$ if /bin/true; then echo that is true; fi
that is true
$ if /bin/false; then echo that is true; fi
$

As you can see you run here the programs directly. No additional [] or [[]].

In case if you want to check whether file does not contain a specific string, you can do it as follows.

if ! grep -q SomeString "$File"; then
  Some Actions # SomeString was not found
fi