bash - How to wait on all child (and grandchild etc) process spawned by a script

Question

Context:

Users provide me their custom scripts to run. These scripts can be of any sort like scripts to start multiple GUI programs, backend services. I have no control over how the scripts are written. These scripts can be of blocking type i.e. execution waits till all the child processes (programs that are run sequentially) exit

#exaple of blocking script
echo "START"
first_program 
second_program 
echo "DONE"

or non blocking type i.e. ones that fork child process in the background and exit something like

#example of non-blocking script
echo "START"
first_program &
second_program &
echo "DONE"

What am I trying to achieve?

User provided scripts can be of any of the above two types or mix of both. My job is to run the script and wait till all the processes started by it exit and then shutdown the node. If its of blocking type, case is plain simple i.e. get the PID of script execution process and wait till ps -ef|grep -ef PID has no more entries. Non-blocking scripts are the ones giving me trouble

Is there a way I can get list of PIDs of all the child process spawned by execution of a script? Any pointers or hints will be highly appreciated

Answer 1

ps --ppid $PID will list all child processes of the process with $PID.