Do bash scripts execute in new shells or subshells?

I am running a bash script from my bash interactive shell as:

./shell.sh

The confusion I am having is, will this script run inside a new shell instance or a subshell of my current bash instance? I assume that all shell scripts invoked from a shell run inside a new shell therefore they aren't able to read the local shell variables of the invoking shell. Also, if I put "echo $BASH_SUBSHELL" in my invoked script it returns me a value of "0" showing that it isn't a subshell. But according to some articles they say that a shell script when executed from a shell invokes a subshell. Please help.

Solution 1:

You're correct; when you run a script with ./shell.sh, it runs in a new shell, not a subshell of the current shell.

It does run in a subprocess, which is a shell, so it's a tempting and common mistake to say "subprocess+shell=subshell, so it must be a subshell!" But that's incorrect. The shell running the script won't inherit shell variables from the parent shell process (it'll inherit environment variables, i.e. exported variables, but that's true of any subprocess), it won't inherit shell modes (e.g. set -e) or other shell state, and it won't even necessarily be running the same shell (if you're running bash and the script has a #!/bin/zsh shebang, it'll run in zsh). So it's logically a different shell that just happens to be running as a subprocess of the shell that launched it.