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How many shells deep I am?

Solutons:


When I read your question, my first thought was $SHLVL
Then I saw that you wanted to count vim levels
in addition to shell levels. 
A simple way to do this is to define a shell function:

vim()  { ( ((SHLVL++)); command vim  "$@");}

This will automatically and silently increment SHLVL
each time you type a vim command. 
You will need to do this for each variant of vi/vim that you ever use; e.g.,

vi()   { ( ((SHLVL++)); command vi   "$@");}
view() { ( ((SHLVL++)); command view "$@");}

The outer set of parentheses creates a subshell,
so the manual change in the value of SHLVL
doesn’t contaminate the current (parent) shell environment. 
Of course the command keyword is there to prevent the functions
from calling themselves (which would result in an infinite recursion loop). 
And of course you should put these definitions
into your .bashrc or other shell initialization file.


There’s a slight inefficiency in the above. 
In some shells (bash being one), if you say

(cmd1; cmd2;; cmdn)

where cmdn is an external, executable program
(i.e., not a built-in command), the shell keeps an extra process lying around,
just to wait for cmdn to terminate. 
This is (arguably) not necessary;
the advantages and disadvantages are debatable. 
If you don’t mind tying up a bit of memory and a process slot
(and to seeing one more shell process than you need when you do a ps),
then do the above and skip to the next section. 
Ditto if you’re using a shell that doesn’t keep the extra process lying around. 
But, if you want to avoid the extra process, a first thing to try is

vim()  { ( ((SHLVL++)); exec vim  "$@");}

The exec command is there to prevent the extra shell process from lingering.

But, there’s a gotcha. 
The shell’s handling of SHLVL is somewhat intuitive:
When the shell starts, it checks whether SHLVL is set. 
If it’s not set (or set to something other than a number),
the shell sets it to 1. 
If it is set (to a number), the shell adds 1 to it.

But, by this logic, if you say exec sh, your SHLVL should go up. 
But that’s undesirable, because your real shell level hasn’t increased. 
The shell handles this by subtracting one from SHLVL
when you do an exec:

$ echo "$SHLVL"
1

$ set | grep SHLVL
SHLVL=1

$ env | grep SHLVL
SHLVL=1

$ (env | grep SHLVL)
SHLVL=1

$ (env) | grep SHLVL
SHLVL=1

$ (exec env) | grep SHLVL
SHLVL=0

So

vim()  { ( ((SHLVL++)); exec vim  "$@");}

is a wash; it increments SHLVL only to decrement it again.
You might as well just say vim, without benefit of a function.

Note:
According to Stéphane Chazelas (who knows everything),
some shells are smart enough not to do this if the exec is in a subshell.

To fix this, you would do

vim()  { ( ((SHLVL+=2)); exec vim  "$@");}

Then I saw that you wanted to count vim levels
independently of shell levels. 
Well, the exact same trick works (well, with a minor modification):

vim() { ( ((SHLVL++, VILVL++)); export VILVL; exec vim "$@");}

(and so on for vi, view, etc.) 
The export is necessary
because VILVL isn’t defined as an environment variable by default. 
But it doesn’t need to be part of the function;
you can just say export VILVL as a separate command (in your .bashrc). 
And, as discussed above, if the extra shell process isn’t an issue for you,
you can do command vim instead of exec vim, and leave SHLVL alone:

vim() { ( ((VILVL++)); command vim "$@");}

Personal Preference:
You may want to rename VILVL to something like VIM_LEVEL
When I look at “VILVL”, my eyes hurt;
they can’t tell whether it’s a misspelling of “vinyl”
or a malformed Roman numeral.


If you are using a shell that doesn’t support SHLVL (e.g., dash),
you can implement it yourself as long as the shell implements a startup file. 
Just do something like

if [ "$SHELL_LEVEL" = "" ]
then
    SHELL_LEVEL=1
else
    SHELL_LEVEL=$(expr "$SHELL_LEVEL" + 1)
fi
export SHELL_LEVEL

in your .profile or applicable file. 
(You should probably not use the name SHLVL, as that will cause chaos
if you ever start using a shell that supports SHLVL.)


Other answers have addressed the issue
of embedding environment variable value(s) into your shell prompt,
so I won’t repeat that, especially you say you already know how to do it.

You could count as many time you need to go up the process tree until you find a session leader. Like with zsh on Linux:

lvl() {
  local n=0 pid=$$ buf
  until
    IFS= read -rd '' buf < /proc/$pid/stat
    set -- ${(s: :)buf##*)}
    ((pid == $4))
  do
    ((n++))
    pid=$2
  done
  echo $n
}

Or POSIXly (but less efficient):

lvl() (
  unset IFS
  pid=$$ n=0
  until
    set -- $(ps -o ppid= -o sid= -p "$pid")
    [ "$pid" -eq "$2" ]
  do
    n=$((n + 1)) pid=$1
  done
  echo "$n"
)

That would give 0 for the shell that was started by your terminal emulator or getty and one more for each descendant.

You only need to do that once on startup. For instance with:

PS1="[$(lvl)]$PS1"

in your ~/.zshrc or equivalent to have it in your prompt.

tcsh and several other shells (zsh, ksh93, fish and bash at least) maintain a $SHLVL variable which they increment on startup (and decrement before running another command with exec (unless that exec is in a subshell if they’re not buggy (but many are))). That only tracks the amount of shell nesting though, not process nesting. Also level 0 is not guaranteed to be the session leader.

Use echo $SHLVL. Use the KISS principle. Depending on your program’s complexity, this may be enough.

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