Bash script to write all occurrences of application crash to a text file - linux

Question

I can't find how to structure my while loop properly to log to a text file all crashes of a given Linux application. I would like to get a prompt so I can input the application name and then a loop to watch the pid of the application. If the pid is null I wanted to log the timestamp in a text file and continue the loop. If still in null at the second iteration, don't log anything, keep monitoring until there are other crashes and other logs... and so on until the script stops with CTRL+C.
I've tried multiple variations of this script without luck. I think I need tips on how to think of a "loop structure" to achieve whatever goals...
read -p "What is the name of the application you want to monitor?" appname
pidofapp=`ps -elf | grep "$appname" | grep -v grep | awk '{print $4}'`
pidofappcheck=`ps -elf | grep "$appname" | grep -v grep | awk '{print $4}'`
while :
do
if [[ ! -z "$pidofappcheck" ]] ; then
pidwasnull=true
pidofappcheck=`ps -elf | grep "$appname" | grep -v grep | awk '{print $4}'`
if [[ "$pidofapp" == "$pidofappcheck" ]] ; then
printf "Still monitoring...\n"
sleep 5
elif [[ "$pidwasnull" == true ]] ; then
continue
fi
echo "FAILURE: Crash occurred at: "$(date)" - Crashes will be logged in the monitoring tool directory under results.txt"
date >> ./results.txt
fi
done
As it is right now, the script will echo:
What is the name of the application you want to monitor?running
Still monitoring...
FAILURE: Crash occurred at: Wed May 22 01:44:53 EDT 2019 - Crashes will be logged in the monitoring tool directory under results.txt
Still monitoring...
FAILURE: Crash occurred at: Wed May 22 01:44:58 EDT 2019 - Crashes will be logged in the monitoring tool directory under results.txt
Thanks in advance for any help.

Answer 1

Try something like this
#!/bin/bash
getpidofapp() {
# pid=$(ps -elf | grep "$1" | grep -v grep | awk '{print $4}' | head -1)
pid=$(pgrep "$1" | head -1)
test -n "${pid}" || { echo "Is ${appname} running?"; exit 1; }
}
read -rp "What is the name of the application you want to monitor?" appname
app_pid=$(getpidofapp "${appname}")
while : ; do
lastpid=$(getpidofapp "${appname}")
if [[ "${app_pid}" == "${lastpid}" ]] ; then
printf "Still monitoring...\n"
else
crashtxt="Crashes will be logged in the monitoring tool directory under results.txt"
echo "FAILURE: Crash occurred at: $(date) ${crashtxt}"
date >> ./results.txt
fi
sleep 5
done

Answer 2

So I have been able to find a solution based on what #Walter A wrote. Here is what I've used. It's working as expected so far.
#!/bin/bash
read -rp "What is the name of the application you want to monitor?" appname
app_pid=$(pidof "$appname")
#echo "First PID of "$appname" is "$app_pid""
while : ; do
lastpid=$(pidof "$appname")
if [[ "${app_pid}" == "${lastpid}" ]] ; then
printf "Still monitoring...\n"
else
crashtxt="Crashes will be logged in the monitoring tool directory under results.txt"
echo "FAILURE: Crash occurred at: $(date) ${crashtxt}"
date >> ./results.txt
app_pid="$lastpid"
fi
sleep 5
done
So this script will basically check the PID of the given app until you CTRL+C. If the PID of the app changes while the script is running. It will output the timestamps of when it occurred in a "results.txt" file and it will keep checking it until you press CTRL+C. Therefore I am going to use this to log all crash occurrences of my apps. Thanks a lot #Walter A