How to start a dummy process in bash which will start another seven descendant processes

Question

I'm trying to write a bash script which would log all descendants process information in below format but I have to log the info only when a process has more than six descendants process.

PID	PPID	Level
10	9	1 (indicates child)
9	8	2 (indicates parent)
8	7	3 (indicates grand parent)

Pstree -ps 10 should display:
dummy1---dummy2---dummy3---dummy4---dummy5---dummy6---dummy7

How to trigger dummy process in above manner?

Frankly I have no idea about bash script, only thing i was able to do was display PID's of a particular user.

while read -r processId;
do
    echo $processId
done <<< "$(ps -u $USER -o pid)"

I need to loop each PID & check if it has more than six descendants but before that I want to manually trigger six descendants process to start & test my script.

Answer 1

To start a script with seven descendants, you can use something like this:

#!/bin/bash

if [[ "$#" -ne 1 ]]; then
    set -- 7
fi

if [[ "$1" -gt 2 ]]; then
    "$0" "$(($1 - 1))"
else
    sleep 120
fi

or more compactly,

#!/bin/bash

[[ "$#" -ne 1 ]] && set -- 7

[[ $1 -gt 2 ]] && $0 $(($1 - 1)) || sleep 120

If this is given no or too many arguments, it sets its argument to 7. Then as long as its argument is greater than 2, it starts a child running the same script, with a decremented argument. When the argument reaches 2, it runs a two-minute sleep. (The limit is two because the last script and the sleep also count as one each.)

$ ./dummy &
$ pstree -p 1800554
dummy(1800554)───dummy(1800561)───dummy(1800563)───dummy(1800566)───dummy(1800567)───dummy(1800570)───sleep(1800572)

You can request a different number of children by changing the argument:

$ ./dummy 6 &

This can be extended to start multiple children in each “generation”:

#!/bin/bash

[[ "$#" -ne 2 ]] && set -- 1 7

for (( i = 0; i < $1; i++ )); do
[[ $2 -gt 2 ]] && $0 $1 $(($2 - 1)) || sleep 120 &
done
wait

Run it as

./dummy 3 2 &

to start two generations with three children each.