How do I know if I'm running a nested shell?

Question

When using a *nix shell (usually bash), I often spawn a sub-shell with which I can take care of a small task (usually in another directory), then exit out of to resume the session of the parent shell.

Once in a while, I'll lose track of whether I'm running a nested shell, or in my top-level shell, and I'll accidentally spawn an additional sub-shell or exit out of the top-level shell by mistake.

Is there a simple way to determine whether I'm running in a nested shell? Or am I going about my problem (by spawning sub-shells) in a completely wrong way?

Answer 1

The $SHLVL variable tracks your shell nesting level:

$ echo $SHLVL 1 $ bash $ echo $SHLVL 2 $ exit $ echo $SHLVL 1 

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As an alternative to spawning sub-shells you could push and pop directories from the stack and stay in the same shell:

[root@localhost /old/dir]# pushd /new/dir /new/dir /old/dir [root@localhost /new/dir]# popd /old/dir [root@localhost /old/dir]# 

Answer 2

Here is a simplified version of part of my prompt:

PS1='$(((SHLVL>1))&&echo $SHLVL)\$ ' 

If I'm not in a nested shell, it doesn't add anything extra, but it shows the depth if I'm in any level of nesting.