How do I test for a non-zero exit status of a command in Bash?

Question

I have a script that checks the exit status of the following function:

function is_git_repository {                                                    
    git branch &> /dev/null                                                     
}  

Which returns 0 if you're in a git repo, and 128 if you're not.

I have no problem testing to see if the return value is 0; the following works as expected:

if is_git_repository ; then 
    echo you are in a git repo
else 
    echo you are NOT in a git repo
fi

But it's when I'm trying to test for an exit status that is anything OTHER than 0 when I'm running into problems. I've tried the following, but none of them work:

1. if [[ "$(is_git_repository)" != "0" ]] ; ... always evaluates to true (link)
2. if [[ "$(is_git_repository)" -ne "0" ]] ; ... always evaluates to false
3. if [[ "$(is_git_repository)" != 0 ]] ; ... always evaluates to true
4. if [[ "$(is_git_repository)" -ne 0 ]] ; ... always evaluates to false
5. if [[ ! "$(is_git_repository)" ]] ; ... always evaluates to true
6. if !is_git_repository ; ... just echoes the command back to me, but without the bang (wtf?)

What is the correct way to check for a non-zero exit status of a command in an if statement?

Answer 1

I soon figured out that if ! is_git_repository ; then ... works as intended (look under 7.1.2.1. Testing exit status in Introduction to if), but why? I would have expected that #1 would work at the very least, but I don't know why it doesn't.

Also, what is up with #6?!

Answer 2

Consider boolean shortcut instead of an if statement:

is_git_repository || echo you are NOT in a git repo