Need an in-depth explanation of how to use flock in Linux shell scripting

Question

I am working on a tiny Raspberry Pi cluster (4 pis). I have 3 Raspberry Pi nodes that will be leaving a message in a message.txt file on the head Pi. The head Pi will be in a loop checking the message.txt file to see if it has any lines. When it does I want to lock the file and then extract the info I need. The problem I am having is that I need to do multiple commands. The only ways I have found that allows multiple commands look like this...

(
flock -s 200

# ... commands executed under lock ...

) 200>/var/lock/mylockfile 

The problem with this way is that it uses a sub shell. The problem with that is that I have "job" files labeled job_1 job_2 etc..... that I want to be able to use a counter with. If I place the increment of the counter in the subshell it will be considered only in the scope of the subshell. If I pull the incrementation out there is a chance that another pi will add an entry before I increment the counter and lock the file.

I have heard talk that there is a way to lock the file and run multiple commands and flow control and then unlock it all using flock. I have not seen any good examples though.

Here is my current code.

# Now go into loop to send out jobs as pis ask for more work
while [ $jobsLeftCount -gt 0 ]
do
echo "launchJobs.sh: About to check msg file"

msgLines=$(wc -l < $msgLocation)
if [ $msgLines ]; then
#FIND WAY TO LOCK FILE AND DO THAT HERE
echo "launchJobs.sh: Messages found. Locking message file to read contents"

(
flock -e 350
echo "Message Received"

while read line; do  
#rename file to be sent to node "job"
mv $jobLocation$jobName$jobsLeftCount /home/pi/algo2/Jobs/job
#transfer new job to each script that left a message
scp /home/pi/algo2/Jobs/job pi@192.168.0.$line:/home/pi/algo2/Jobs/
jobsLeftCount=$jobsLeftCount-1;
echo $line 
done < $msgLocation


#clear msg file
>$msgLocation
#UNLOCK MESG FILE HERE
) 350>>$msgLocation


echo "Head node has $jobsLeftCount remaining"


fi
#jobsLeftCount=$jobsLeftCount-1;
#echo "here is $jobsLeftCount file"
done

Answer 1

If the sub-shell environment is not acceptable, use braces in place of parentheses to group the commands:

{
flock -s 200

# ... commands executed under lock ...

} 200>/var/lock/mylockfile

This runs the commands executed under lock in a new I/O context, but does not start a sub-shell. Within the braces, all the commands executed will have file descriptor 200 open to the locked lock file.