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

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

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 onthe result of search. 0 if something was found; 1 otherwise.

$ echo hello | grep hi ; echo $?1$ echo hello | grep he ; echo $?hello0$ 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; fithat 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 foundfi

In addition to other answers, which told you how to do what you wanted, I try to explain what was wrong (which is what you wanted.

In Bash, if is to be followed with a command. If the exit code of this command is equal to 0, then the then part is executed, else the else part if any is executed.

You can do that with any command as explained in other answers: if /bin/true; then ...; fi

[[ is an internal bash command dedicated to some tests, like file existence, variable comparisons. Similarly [ is an external command (it is located typically in /usr/bin/[) that performs roughly the same tests but needs ] as a final argument, which is why ] must be padded with a space on the left, which is not the case with ]].

Here you needn't [[ nor [.

Another thing is the way you quote things. In bash, there is only one case where pairs of quotes do nest, it is "$(command "argument")". But in 'grep 'SomeString' $File' you have only one word, because 'grep ' is a quoted unit, which is concatenated with SomeString and then again concatenated with ' $File'. The variable $File is not even replaced with its value because of the use of single quotes. The proper way to do that is grep 'SomeString' "$File".