`if expr` vs `if [[ expr ]]`?

[progrium]

`if expr` vs `if [[ expr ]]`?

[progrium]

Depends on what expr is. You can do if expr to run a command and use the exit status. [[ is for other non-exit code evaluations.

[shazow]

Aha, that's the one! So for example, you're better off doing something like...

# Best
if grep -q 'foo' somefile; then
  ...
fi

# Won't work
if [ grep -q 'foo' somefile ]; then
  ...
fi

# Will work, but less nice
if [ "$(grep -m1 'foo' somefile)" ]; then
  ...
fi

Worth adding something about this to the bashstyle?

[progrium]

Sure. "If you're just testing exit code, don't use [[" ... (or [, but that's already forbidden)

Your last example isn't just less nice, it's semantically different. It will test if the output is not empty. Sometimes that's the same, sometimes not.