How do I know how many sub-shells deep I am?

You can use the command pstree (that comes by default with Ubuntu). Here is example - currently I'm having only one open terminal window on WSL:

User@Wsl:~$ pstree
init─┬─init───bash───pstree
     └─{init}

User@Wsl:~$ bash
User@Wsl:~$ sh
$ bash
User@Wsl:~$ pstree
init─┬─init───bash───bash───sh───bash───pstree
     └─{init}

Within an actual Linux/Ubuntu environment the process tree will be more complicated. We can filter the tree by the option -s that will show the parents of a selected process. So our command could be pstree -s $$, where $$ is an environment variable that contains the current PID:

User@Ubuntu:~$ pstree -s $$
systemd──lightdm──lightdm──upstart──gnome-terminal-──bash──pstree

User@Ubuntu:~$ bash
User@Ubuntu:~$ sh
$ bash
User@Ubuntu:~$ pstree -s $$
systemd──lightdm──lightdm──upstart──gnome-terminal-──bash──bash──sh──bash──pstree

References:

  • SuperUser: How to get parent PID of a given process in GNU/Linux from command line?
  • HowtoForge: Linux pstree Command Tutorial for Beginners

Add indicator to the shell's prompt: Based on the @waltinator's idea, in order to have a counter in the front of the prompt for several different shells when the level is deeper than one, I've added the lines, shown below the demo, at the bottom of the relevant run commands (~/.*rc) files.

I've made tests on WSL, Ubuntu 16.04, Ubuntu 18.04 (server/desktop), Ubuntu 19.04, within gnome-terminal, tty and ssh session. Here is how this works:

enter image description here

The limitation is that: the counter works only for 13-14 levels of depth, depending on the OS. I do not intend to investigate the reasons :)

  • bash > .bashrc:

    DEPTH=$(($(pstree -s $$ | sed -r 's/-+/\n/g' | grep -Ec '\<(bash|zsh|sh|dash|ksh|csh|tcsh)\>') - 1))
if (( DEPTH > 1 )); then PS1=$DEPTH:$PS1; fi

  • csh and tcsh > .cshrc:

    @ DEPTH = `pstree -s $$ | sed -r 's/-+/\n/g' | grep -Ec '\<(bash|zsh|sh|dash|ksh|csh|tcsh)\>'` - 0
if ( $DEPTH > 1 ) then; set prompt="$DEPTH":"$prompt"; endif

  • zsh > .zshrc:

    DEPTH=$(($(pstree -s $$ | sed -r 's/-+/\n/g' | grep -Ec '\<(bash|zsh|sh|dash|ksh|csh|tcsh)\>') - 1))
if (( DEPTH > 1 )); then PROMPT=$DEPTH:$PROMPT; fi

  • ksh > .kshrc:

    DEPTH=$(($(pstree -s $$ | sed -r 's/\-+/\n/g' | grep -Ec '\<(bash|zsh|sh|dash|ksh|csh|tcsh)\>') - 0))
if (( DEPTH > 1 )); then PS1="$DEPTH":"$PS1"'$ '; fi

  • sh that is actually dash on Ubuntu - here the things are little bit complicated and wired (read the references below for more information):

    1. Edit the ~/.profile file and add the following line at the bottom:

      ENV=$HOME/.shrc; export ENV

    2. Create the file ~/.shrc with the following content, note ksh also reads the $ENV:

      #!/bin/dash
DEPTH=$(pstree -s $$ | sed -r 's/-+/\n/g' | grep -Ec '\<(bash|zsh|sh|dash|ksh|csh|tcsh)\>')
if [ "$0" != 'ksh' ]; then DEPTH=$((DEPTH - 1)); fi
if [ "$DEPTH" -gt 1 ]; then export PS1='$DEPTH:\$ '; fi

References:

  • The previous version of the answer, where the counter works only in Bash
  • Unix&Linux: Is there an invoked file for dash and sh as non login shells in /etc directory?
  • Ubuntu Manuals: Dash (read the BUGS section)
  • UnixWare 7 Documentation: The Korn shell .profile and .kshrc

Create a command that will output the depth: Another option is to create shell command that will output the depth. For this purpose create the executable file /usr/local/bin/depth (thus it should be accessible system wide):

sudo touch /usr/local/bin/depth
sudo chmod +x /usr/local/bin/depth

Edit the file with your favorite editor and add the following lines as its content:

#!/bin/bash

SHELLS='(bash|zsh|sh|dash|ksh|csh|tcsh)'
DEPTH=$(pstree -s $$ | sed -r 's/-+/\n/g' | grep -Ec "\<$SHELLS\>")

if [[ $@ =~ -v ]]
then
        pstree -s $$ | sed -r 's/-+/\n/g' | grep -E "\<$SHELLS\>" | cat -n
fi

echo "DEPTH: $DEPTH"

[[ $DEPTH -gt 1 ]] && exit 0 || exit 1

The above script has two options -v or --verbose that will output a list of the involved shells. And the another option that will check whether the depth is greater than one and based on this will return exit 0 or exit 1, so you can use it in this way depth && exit. Here are few examples of usage:

User@Ubuntu:~$ depth          # we are at the 1st level - bash
DEPTH: 1
User@Ubuntu:~$ sh           
$ csh                         # we are at the 2nd level - dash
Ubuntu:~% depth               # we are at the 3rd level - csh
DEPTH: 3
Ubuntu:~% ksh
$ depth -v                    # we are at the 4th level - ksh
     1  bash
     2  sh
     3  csh
     4  ksh
DEPTH: 4
$ depth && exit               # exit to the 3rd level - csh
DEPTH: 4
Ubuntu:~% depth && exit       # exit to the 2nd level - dash
DEPTH: 3
exit
$ depth && exit               # exit to the 1st level - bash
DEPTH: 2
User@Ubuntu:~$ depth && exit  # stay at the 1st level - bash
DEPTH: 1
User@Ubuntu:~$ depth && exit  # stay at the 1st level - bash
DEPTH: 1

Comparison by the other solutions: I spent some additional time to find out some weaknesses of the approaches provided here. I was able to imagine the following two cases (the capital letters are needed for better syntax highlighting):

  • When su or sudo -i are involved:

    User@Ubuntu:~$ ps | grep -Ec '\<(bash|zsh|sh|dash|ksh|csh|tcsh|su|sudo)\>'
1
User@Ubuntu:~$ echo $SHLVL
1
User@Ubuntu:~$ depth
DEPTH: 1

User@Ubuntu:~$ su spas
Password:

Spas@Ubuntu:~$ ps | grep -Ec '\<(bash|zsh|sh|dash|ksh|csh|tcsh|su|sudo)\>'
1
Spas@Ubuntu:~$ echo $SHLVL
2
Spas@Ubuntu:~$ depth
DEPTH: 2

Spas@Ubuntu:~$ sudo -i
[sudo] password for spas:

Root@Ubuntu:~# ps | grep -Ec '\<(bash|zsh|sh|dash|ksh|csh|tcsh|su|sudo)\>'
3
Root@Ubuntu:~# echo $SHLVL
1
Root@Ubuntu:~# depth
DEPTH: 3

  • When there a background process is launched:

    User@Ubuntu:~$ bash
User@Ubuntu:~$ ps | grep -Ec '\<(bash|zsh|sh|dash|ksh|csh|tcsh)\>'
2
User@Ubuntu:~$ echo $SHLVL
2
User@Ubuntu:~$ depth
DEPTH: 2

User@Ubuntu:~$ while true; do sleep 10; done &
[1] 10886
User@Ubuntu:~$ ps | grep -Ec '\<(bash|zsh|sh|dash|ksh|csh|tcsh)\>'
3
User@Ubuntu:~$ echo $SHLVL
2
User@Ubuntu:~$ depth
DEPTH: 2

# Note: $SHLVL is not supported only by sh/dash.  
#       It works with all other tested shells: bash, zsh, csh, tcsh, ksh

User@Ubuntu:~$ sh
$ ps | grep -Ec '\<(bash|zsh|sh|dash|ksh|csh|tcsh)\>'
4
$ echo $SHLVL
2
$ depth
DEPTH: 3

Check the value of the SHLVL shell variable:

echo $SHLVL

Quoting from bash's manual page:

SHLVL  Incremented by one each time an instance of bash is started.

It is also supported by zsh.


In my .bashrc, I use $SHLVL to adjust $PS1, by appending "+" signs to my $SUBSHELL variable: 

...
# set a variable to reflect SHLVL > 1 (Ubuntu 12.04)
if [[ $SHLVL -gt 1 ]] ; then
    export SUBSHELL="${SUBSHELL:+$SUBSHELL}+"
else
    export SUBSHELL=""
fi
...

if [[ "$color_prompt" = yes ]]; then
#             chroot?                       Depth      green       user@host nocolor  :   green      $PWD  red      (status) off   $ or # space             
    PS1='${debian_chroot:+($debian_chroot)}${SUBSHELL}\[\033[01;32m\]\u@\h\[\033[00m\]:\[\033[01;34m\]\w\[\033[1;31m\]($?)\[\033[00m\]\$ '
else
    PS1='${debian_chroot:+($debian_chroot)}${SUBSHELL}\u@\h:\w\$ '
fi
...

Then, I can see how deep I am: 

walt@bat:~(1)$ ed foo
263
!bash
+walt@bat:~(0)$ bash
++walt@bat:~(0)$ bash
+++walt@bat:~(0)$ exit
exit
++walt@bat:~(0)$ exit
exit
+walt@bat:~(0)$ exit
exit
!
q
walt@bat:~(0)$

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