Does not work to execute command in double brackets in bash

Question

In an attempt to stay consistent i have tried to use double brackets [[ ]] in all my if statements. I did however get into a problem when i was going to check the return value f

Answer 1

Double braces are a shortcut for test. In your examples, what's happening is that you're testing the shell variable $command for existence.

if [[ $PWD ]]; then
    echo PWD is set to a value
fi

if [[ $NOT_A_REAL_VAR ]]; then
    echo Nope, its not set
fi

In your second example, you're using command substitution to check that command output something on standard output.

if [[ $(echo hi) ]]; then
    echo "echo said hi'
fi

if [[ $(true) ]]; then #true is a program that just quits with successful exit status
    echo "This shouldn't execute"
fi

Your third example is the same as your first, pretty much. You use the curly braces if you want to group your variables. for example if you want to put an 's' after something.

WORD=Bike
echo "$WORDS" #won't work because "WORDS" isn't a variable
echo "${WORD}S" # will output "BikeS"

Then in your fifth example, you are running the program that is sitting inside command.

So, if you want to test some strings, use [[ ]] or [ ]. If you just want to test the exit status of a program, then don't use those, just use a bare if.

Check man test for details on the braces.