Identifying shell script's subshell number from within a script itself

Question

I am accidentally caught in a desire to reveal the subshell number (BASH_SUBSHELL) from within the script itself, but i get subshell 0

Here is the script's line

echo "Operated from subshell: $BASH_SUBSHELL

Part of the shell's output in terminal

• echo 'Operated from subshell: 0' Operated from subshell: 0

Question Is it possible to reveal the subshell a script is operating from within the script itself?

Answer 1

Since scripts do not run in subshells, this output is correct. Subshells are created by a few things, including parentheses ( ... ), backgrounding with &, and command substitution $( ... ), but not by launching scripts: that creates a whole new shell to execute the script in.

What you may be thinking of is the SHLVL variable, which does increment for each layer of script (and shell):

SHLVL Incremented by one each time a new instance of Bash is started. This is intended to be a count of how deeply your Bash shells are nested.

If your script line were

echo "Operated from shell level: $SHLVL"

then you would get the output that I think you expected. If the script recursed, $SHLVL would increment each time.

Answer 2

Here's some code that actually uses some subhells:

echo "main shell: $BASH_SUBSHELL"
( 
    echo "first subshell: $BASH_SUBSHELL"
    ( 
        echo "second subshell: $BASH_SUBSHELL"
        (
            echo "third subshell: $BASH_SUBSHELL"
        )
    )
)