Efficiently find PIDs of many processes started by services - bash

Question

I have a file with many service names, some of them are running, some of them aren't.
foo.service
bar.service
baz.service
I would like to find an efficient way to get the PIDs of the running processes started by the services (for the not running ones a 0, -1 or empty results are valid).
Desired output example:
foo.service:8484
bar.server:
baz.service:9447
(bar.service isn't running).
So far I've managed to do the following: (1)
cat t.txt | xargs -I {} systemctl status {} | grep 'Main PID' \
| awk '{print $3}'
With the following output:
8484
9447
But I can't tell which service every PID belongs to.
(I'm not bound to use xargs, grep or awk.. just looking for the most efficient way).
So far I've managed to do the following: (2)
for f in `cat t.txt`; do
v=`systemctl status $f | grep 'Main PID:'`;
echo "$f:`echo $v | awk '{print \$3}'`";
done;
-- this gives me my desired result. Is it efficient enough?

Answer 1

I ran into similar problem and fount leaner solution:
systemctl show --property MainPID --value $SERVICE
returns just the PID of the service, so your example can be simplified down to
for f in `cat t.txt`; do
echo "$f:`systemctl show --property MainPID --value $f`";
done

Answer 2

You could also do:
while read -r line; do
statuspid="$(sudo service $line status | grep -oP '(?<=(process|pid)\s)[0-9]+')"
appendline=""
[[ -z $statuspid ]] && appendline="${line}:${statuspid}" || appendline="$line"
"$appendline" >> services-pids.txt
done < services.txt
To use within a variable, you could also have an associative array:
declare -A servicearray=()
while read -r line; do
statuspid="$(sudo service $line status | grep -oP '(?<=(process|pid)\s)[0-9]+')"
[[ -z $statuspid ]] && servicearray[$line]="statuspid"
done < services.txt
# Echo output of array to command line
for i in "${!servicearray[#]}"; do # Iterating over the keys of the associative array
# Note key-value syntax
echo "service: $i | pid: ${servicearray[$i]}"
done
Making it more efficient:
To list all processes with their execution commands and PIDs. This may give us more than one PID per command, which might be useful:
ps -eo pid,comm
So:
psidcommand=$(ps -eo pid,comm)
while read -r line; do
# Get all PIDs with the $line prefixed
statuspids=$(echo $psidcommand | grep -oP '[0-9]+(?=\s$line)')
# Note that ${statuspids// /,} replaces space with commas
[[ -z $statuspids ]] && appendline="${line}:${statuspids// /,}" || appendline="$line"
"$appendline" >> services-pids.txt
done < services.txt
OUTPUT:
kworker:5,23,28,33,198,405,513,1247,21171,22004,23749,24055
If you're confident your file has the full name of the process, you can replace the:
grep -oP '[0-9]+(?=\s$line)'
with
grep -oP '[0-9]+(?=\s$line)$' # Note the extra "$" at the end of the regex
to make sure it's an exact match (in the grep without trailing $, line "mys" would match with "mysql"; in the grep with trailing $, it would not, and would only match "mysql").

Answer 3

Building up on Yorik.sar's answer, you first want to get the MainPID of a server like so:
for SERVICE in ...<service names>...
do
MAIN_PID=`systemctl show --property MainPID --value $SERVICE`
if test ${MAIN_PID} != 0
than
ALL_PIDS=`pgrep -g $MAIN_PID`
...
fi
done
So using systemctl gives you the PID of the main process controlled by your daemon. Then the pgrep gives you the daemon and a list of all the PIDs of the processes that daemon started.
Note: if the processes are user processes, you have to use the --user on the systemctl command line for things to work:
MAIN_PID=`systemctl --user show --property MainPID --value $SERVICE`
Now you have the data you are interested in the MAIN_PID and ALL_PIDS variables, so you can print the results like so:
if test -n "${ALL_PID}"
then
echo "${SERVICE}: ${ALL_PIDS}"
fi