systemd unit check if another service is ready before launching

is-service-up.sh

#!/bin/bash
# Shell script for a systemd unit to figure out if a service is already
# running.
# Useful to check on ExecPre if a required service is ready, and, if not,
# wait until it is. Particularly effective to check for MySQL/MariaDB.
#
# Usage:
# `is-service-up.sh [service name] [sleeping time in seconds] [grep search string to check]
# where only $1 is mandatory, the rest optional
# e. g. `is-service-up.sh some.service 60 ' active'`
# Note that this script cannot return any output, which will be ignored by
# systemctl.
# (gwyneth 20211118)
# v1.1 - Added more sophisticated check for empty/numeric time (it was
# failing with the defaults some times)
# See https://stackoverflow.com/a/3951175/1035977
# (gwyneth 20260531)

# Extract parameters, if they exist
# At least, we need the service name; abort otherwise with error 22 (invalid
# argument) (gwyneth 20211118)
if [ $# -eq 0 ]; then exit 22; fi

# echo "Arguments: $1 $2"
# init variables with command-line arguments
serviceName="$1"
sleepTime="$2"

#if [ -z $sleepTime ]
#then
#   sleepTime=60
#fi

case "$sleepTime" in
    ''|*[!\-\.0-9]*|?*-*|*.*.*) sleepTime=60 ;;
esac

# echo "serviceName=$serviceName and sleepTime=$sleepTime"

# First, check that the service is active and enabled.
# is-enabled is not totally quiet (even with --quiet) so we redirect stderr
# to /dev/null
function is_service_active {
#   echo "Inside function call, launching command"
    /usr/bin/systemctl --quiet is-active $serviceName || /usr/bin/systemctl --quiet is-enabled $serviceName > /dev/null 2
#   /usr/bin/systemctl status $serviceName | /usr/bin/grep $searchString > /dev/null 2>&1
    local retvalue=$?
#   echo "Inside function call, result was: "$retvalue
    return $retvalue
}

# Debugging... (one day, it will be a command option...)
#echo "Service: $serviceName $sleepTime"
#
#is_service_active
#if [ $? -eq 0 ]
#then
#   echo "Active, status was $?"
#else
#   until [ $? -eq 0 ]
#   do
#       echo "Got $?; sleeping now for $sleepTime..."
#       sleep $sleepTime
#       is_service_active
#   done
#fi

# Loop until service becomes active; sleep a bit between checks
is_service_active
until [ $? -eq 0 ]
do
#   echo "Got $?"
    sleep $sleepTime
    is_service_active
done