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You have already posted another question on this same topic two weeks later, with a better solution using GNU parallel. I'll review this one too anyway on its own merit, though it might be a bit of a moot point.

It's not a good idea to set StrictHostKeyChecking=no when using ssh. The host key of servers should normally not change. When they do, and you don't know why, it might be a man in the middle attack. If it's part of a scheduled server update, you can manually update the ~/.ssh/known_hosts file accordingly.

When filtering the output of ls like this:

ls -dt1 path | head -n1

you don't need the -1 flag. The purpose of that flag is to print the list of files in a single column. But when you pipe the output to another command, the output will be always a single column.

Instead of this:

if [ "$dir1" = "$dir2" ]

This is easier (because you don't need to quote the variables) and more modern:

if [[ $dir1 = $dir2 ]]

Instead of cramming so many ssh options on the command line:

scp -o ControlMaster=auto -o 'ControlPath=~/.ssh/control-%r@%h:%p' -o ControlPersist=900 david@machineA:...

it's better to add these options in the ~/.ssh/config file, like this:

Host machineA
Hostname machineA
User david
ControlMaster auto
ControlPath ~/.ssh/control-%r@%h:%p
ControlPersist 900

This way the scp command becomes simply:

scp machineA:...

I hope this (and even more, my other answer) helps!

You have already posted another question on this same topic two weeks later, with a better solution using GNU parallel. I'll review this one too anyway on its own merit, though it might be a bit of a moot point.

It's not a good idea to set StrictHostKeyChecking=no when using ssh. The host key of servers should normally not change. When they do, and you don't know why, it might be a man in the middle attack. If it's part of a scheduled server update, you can manually update the ~/.ssh/known_hosts file accordingly.

When filtering the output of ls like this:

ls -dt1 path | head -n1

you don't need the -1 flag. The purpose of that flag is to print the list of files in a single column. But when you pipe the output to another command, the output will be always a single column.

Instead of this:

if [ "$dir1" = "$dir2" ]

This is easier (because you don't need to quote the variables) and more modern:

if [[ $dir1 = $dir2 ]]

Instead of cramming so many ssh options on the command line:

scp -o ControlMaster=auto -o 'ControlPath=~/.ssh/control-%r@%h:%p' -o ControlPersist=900 david@machineA:...

it's better to add these options in the ~/.ssh/config file, like this:

Host machineA
Hostname machineA
User david
ControlMaster auto
ControlPath ~/.ssh/control-%r@%h:%p
ControlPersist 900

This way the scp command becomes simply:

scp machineA:...

I hope this (and even more, my other answer) helps!