How to get subshell's PID in Korn Shell (equivalent of $BASHPID)

Question

In bash you have this handy variable: $BASHPID wich always returns the currently running subshell's PID. How can I get a subshell's PID in ksh? For example see the code below:

#!/usr/bin/ksh93

echo "PID at start: $$"

function run_in_background {
  echo "PID in run_in_background $$"
  run_something &
  echo "PID of backgrounded run_something: $!"
}

function run_something {
  echo "*** PID in run_something: $$"
  sleep 10;
}    

run_in_background
echo "PID of run in background $!"

This outputs the following:

PID at start: 5328
PID in run_in_background 5328
*** PID in run_something: 5328
PID of backgrounded run_something: 5329
PID of run in background 5329

What I want is the line starting with **** to output the subshell's PID, in the example's case that would be 5329.

Answer 1

I don't think that's available in ksh. There's a POSIX solution which involves running an external process:

sh -c 'echo $PPID'

On Linux, readlink /proc/self would also work, but I fail to see any advantage (it might be marginally faster; it could be useful on a BusyBox variant that has readlink but not $PPID, but I don't think there is one).

Note that in order to get the value in the shell, you need to be careful not to run that command in a short-lived sub-sub-shell. For example, p=$(sh -c 'echo $PPID') might show the output of the subshell that invokes sh within the command substitution (or it might not, some shells optimize that case). Instead, run

p=$(exec sh -c 'echo $PPID')

Answer 2

You can achieve what you want, but you need to put run_something into a separate script. I'm not exactly sure why, but $$ is not re-evaluated when it is used in a function in the same script that is calling it. I guess that the value of $$ is assigned once after the script is parsed and before it is executed.

run_in_background.sh

#
echo "PID at start: $$"

    function run_in_background {
      echo "PID in run_in_background $$"
      ./run_something.sh &
      echo "PID of backgrounded run_something: $!"
    }

    run_in_background
    echo "PID of run in background $!"

run_something.sh

#
echo "*** PID in run_something: $$"
sleep 10;

output

PID at start: 24395
PID in run_in_background 24395
PID of backgrounded run_something: 24396
PID of run in background 24396
*** PID in run_something: 24396

Answer 3

# KSH_VERSION hasn't always been a nameref, nor has it always existed.
# Replace with a better test if needed. e.g.:
# https://www.mirbsd.org/cvs.cgi/contrib/code/Snippets/getshver?rev=HEAD
if [[ ${!KSH_VERSION} == .sh.version ]]; then
    # if AT&T ksh
    if builtin pids 2>/dev/null; then # >= ksh93 v- alpha
        function BASHPID.get { .sh.value=$(pids -f '%(pid)d'); }
    elif [[ -r /proc/self/stat ]]; then # Linux / some BSDs / maybe others
        function BASHPID.get { read -r .sh.value _ </proc/self/stat; }
    else # Crappy fallback
        function BASHPID.get { .sh.value=$(exec sh -c 'echo $PPID'); }
    fi
elif [[ ! ${BASHPID+_} ]]; then
   echo 'BASHPID requires Bash, ksh93, or mksh >= R41' >&2
   exit 1
fi