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

The $SHLVL variable tracks your shell nesting level:

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

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]#

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.

Look at $0: if it starts with a minus -, you're in the login shell.

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