Check if bash shell is interactive

check bash interactive.md

The old way

if tty -s; then
  echo "interactive"
fi

Cons

This method does the job, but invloves process invocation (fork/exec) of tty, which is expensive in terms of performance.
But there's a worse side-effect:
If PATH is empty or wrong, calling tty will fail. When sourced for a login shell, bash may emmit an error. Otherwise this may end the shell.

The better way

if [[ $- =~ "i" ]]; then
  echo "interactive"
fi

Pros

The check here uses only bash builtins. It's much safer and performs better.

The better way [2]

case $- in
   *i*) echo "interactive";;
esac

Pros

Similar to the above method, useful if you want to test other set flags.