Linux Service unexpectedly died - linux

Question

Running Ubuntu 17.04, I'd like to have a systemctl service which oversees a main bash script, where three programs (here substituted by dummy script foo_script tagged with an argument) run under an endless loop (because of possible program crashes).
The main script, foo_main.sh, works correctly if called from a command line; but the service I'm trying to set up from it crashes soon.
File foo_script.sh:
#!/bin/bash
echo "FooScripting "$1 >> "foo.d/"$1
File loop.sh:
#!/bin/bash
nLoop=0
prgName=$1
prgArg=$2
echo "<< START of "${prgName} ${prgArg}" loop >>"
while :
do
let nLoop=nLoop+1
echo "<< looping "${prgName} ${prgArg}" >>" ${nLoop}
"./"${prgName} ${prgArg}
sleep 1
done
echo "<< END of "${prgName} ${prgArg}" loop >>"
File foo_main.sh:
#!/bin/bash
echo "foo_main start in "${PWD}
./loop.sh "foo_script.sh" "fb" &
sleep 2
./loop.sh "foo_script.sh" "gc" &
./loop.sh "foo_script.sh" "gb" &
echo "foo_main end"
File /etc/systemd/system/food.service:
[Unit]
Description = Foo Daemon
After = network.target
[Service]
Type = simple
# User = <<USER>>
# PIDFile=/var/food.pid
WorkingDirectory = /home/john/bin
ExecStart = /home/john/bin/foo_main.sh
# ExecStop = killall loop.sh
# ExecReload = killall loop.sh && /home/john/bin/foo_main.sh
# Restart = on-abort
[Install]
WantedBy = multi-user.target
What I obtain from every sudo systemctl status food.service (after a start ofc) is almost the same output
● food.service - Foo Daemon
Loaded: loaded (/etc/systemd/system/food.service; disabled; vendor preset: enabled)
Active: inactive (dead)
Sep 28 14:54:30 john-host systemd[1]: Started Foo Daemon.
Sep 28 14:54:30 john-host foo_main.sh[7376]: foo_main script start in /home/john/bin
Sep 28 14:54:30 john-host foo_main.sh[7376]: << START of foo_script.sh fb loop >>
Sep 28 14:54:30 john-host foo_main.sh[7376]: << looping foo_script.sh fb >> 1
Sep 28 14:54:31 john-host foo_main.sh[7376]: << looping foo_script.sh fb >> 2
Sep 28 14:54:32 john-host foo_main.sh[7376]: foo_main script end
Sep 28 15:24:30 john-host foo_main.sh[7921]: << START of foo_script.sh gb loop >>
Sep 28 15:24:30 john-host foo_main.sh[7921]: << START of foo_script.sh gc loop >>
Sep 28 15:24:30 john-host foo_main.sh[7921]: << looping foo_script.sh gb >> 1
Sep 28 15:24:30 john-host foo_main.sh[7921]: << looping foo_script.sh gc >> 1

Answer 1

Another solution is to use Type=oneshot + RemainAfterExit=yes in your /etc/systemd/system/food.service
Look at https://www.freedesktop.org/software/systemd/man/systemd.service.html to refer onshot type of service.
The service file like following should solve your issue too.
[Unit]
Description = Foo Daemon
After = network.target
[Service]
Type = oneshot
RemainAfterExit=yes
# User = <<USER>>
# PIDFile=/var/food.pid
WorkingDirectory = /home/john/bin
ExecStart = /home/john/bin/foo_main.sh
# ExecStop = killall loop.sh
# ExecReload = killall loop.sh && /home/john/bin/foo_main.sh
# Restart = on-abort
[Install]
WantedBy = multi-user.target

Answer 2

Solved... the service was stopped simply because its execution flow ended with foo_main.sh. It was enough to add something like
# ...
./loop.sh "foo_script.sh" "endless_dummy_loop"
Without background ampersand, at the end of foo_main.sh.
Clearly real services are far different from this, but I got the point now.