SSH to multiple hosts at once

Question

I have a script which loops through a list of hosts, connecting to each of them with SSH using an RSA key, and then saving the output to a file on my local machine - this all works correctly. However, the commands to run on each server take a while (~30 minutes) and there are 10 servers. I would like to run the commands in parallel to save time, but can't seem to get it working. Here is the code as it is now (working):

for host in $HOSTS; do
    echo "Connecting to $host"..
    ssh -n -t -t $USER@$host "/data/reports/formatted_report.sh"
done

How can I speed this up?

Answer 1

You should add & to the end of the ssh call, it will run on the background.

for host in $HOSTS; do
    echo "Connecting to $host"..
    ssh -n -t -t $USER@$host "/data/reports/formatted_report.sh" &
done

Answer 2

I tried using & to send the SSH commands to the background, but I abandoned this because after the SSH commands are completed, the script performs some more commands on the output files, which need to have been created.

Using & made the script skip directly to those commands, which failed because the output files were not there yet. But then I learned about the wait command which waits for background commands to complete before continuing. Now this is my code which works:

for host in $HOSTS; do
    echo "Connecting to $host"..
    ssh -n -t -t $USER@$host "/data/reports/formatted_report.sh" &
done
wait