In Bash, how can I detect if I'm in a subshell?

Question

I'm trying to write a function to replace the functionality of the exit builtin to prevent myself from exiting the terminal.

I have attempted to use the SHLVL environment variable but it doesn't seem to change within subshells:

$ echo $SHLVL
1
$ ( echo $SHLVL )
1
$ bash -c 'echo $SHLVL'
2

My function is as follows:

exit () {
    if [[ $SHLVL -eq 1 ]]; then
        printf '%s\n' "Nice try!" >&2
    else
        command exit
    fi
}

---

This won't allow me to use exit within subshells though:

$ exit
Nice try!
$ (exit)
Nice try!

What is a good method to detect whether or not I am in a subshell?

Answer 1

In bash, you can compare $BASHPID to $$

$ ( if [ "$$" -eq "$BASHPID" ]; then echo not subshell; else echo subshell; fi )
subshell
$   if [ "$$" -eq "$BASHPID" ]; then echo not subshell; else echo subshell; fi
not subshell

If you're not in bash, $$ should remain the same in a subshell, so you'd need some other way of getting your actual process ID.

One way to get your actual pid is sh -c 'echo $PPID'. If you just put that in a plain ( … ) it may appear not to work, as your shell has optimized away the fork. Try extra no-op commands ( : ; sh -c 'echo $PPID'; : ) to make it think the subshell is too complicated to optimize away. Credit goes to John1024 on Stack Overflow for that approach.

Answer 2

How about BASH_SUBSHELL?

BASH_SUBSHELL
      Incremented by one within each subshell or subshell environment when the shell
      begins executing in that environment. The initial value is 0.

$ echo $BASH_SUBSHELL
0
$ (echo $BASH_SUBSHELL)
1

Answer 3

[this should've been a comment, but my comments tend to be deleted by moderators, so this will stay as an answer that I could use it as a reference even if deleted]

Using BASH_SUBSHELL is completely unreliable as it be only set to 1 in some subshells, not in all subshells.

$ (echo $BASH_SUBSHELL)
1
$ echo $BASH_SUBSHELL | cat
0

Before claiming that the subprocess a pipeline command is run in is not a really real subshell, consider this man bash snippet:

Each command in a pipeline is executed as a separate process (i.e., in a subshell).

and the practical implications -- it's whether a script fragment is run a subprocess or not which is essential, not some terminology quibble.

The only solution, as already explained in the answers to this question is to check whether $BASHPID equals $$ or, portably but much less efficient:

if [ "$(exec sh -c 'echo "$PPID"')" != "$$" ]; then
    echo you\'re in a subshell
fi