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I wrote a script that runs commands on 1000+ servers in background. Sometimes the script gets hung on one of the servers. If/when a server gets hung(due to high load avg) when running a script,the command might also gets hung on that server. Is there a way to skip that host so the script can go to the next host and keep running along?.

I am highlighting two main function of my script, but no luck in giving "ConnectTimeout" and wait keywords.

exec_ssh()
{
for i in `cat $file`
do 
    ssh -q -o "StrictHostKeyChecking no" -o "NumberOfPasswordPrompts 0" -o ConnectTimeout=2 $i $command  2>>/dev/null &
        if wait $!; then
                echo "" >> /dev/null
        else
                echo "$i is not reachable over SSH or passwordless authentication is not setup on the server" >> /tmp/not_reachable
        fi

done >/tmp/output.csv &


run_command()
{
                        export -f exec_ssh
                        export command
                        nohup bash -c exec_ssh &>>$log_file &
}
2
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Your script as written would keep running all of your remote commands concurrently, but for your use of wait which explicitly will wait for a backgrounded task to complete. In the case you describe of a high-load server, this means your ssh command is not timing out, but is simply taking a long time to complete, so the script is doing exactly what you ask it to. ConnectTimeout is moot when you are able to successfully make the ssh connection.

If you do want to use this sort of script rather than a tool designed for distributed remote execution such as Ansible, I might modify your script as follows:

exec_ssh() {
  while read file; do
    if ! ssh -q -o BatchMode=yes -o ConnectTimeout=2 "$i" "$command"  2>>/dev/null & then
      echo "$i is not reachable via non-interactive SSH or remote command threw error - exit code $?" >> /tmp/not_reachable
    fi
  done < "$file" > /tmp/output.csv &
}

run_command() {
    export -f exec_ssh
    export command
    nohup bash -c exec_ssh &>> "$log_file" &
}

It also might be worth considering separating your "can I SSH to the host" test and your "can I complete the job" test:

if ssh -q -o BatchMode=yes -o ConnectTimeout=2 "$host" true; then
    # connection succeeded
  if ! ssh -q -o BatchMode=yes -o ConnectTimeout=2 "$host" "$command" & then
    echo "Remote command threw $?"
  fi
else
    echo "SSH threw $?"
fi
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  • thanks, I will try this out and let you know. – Sin15 Jan 23 '19 at 16:38
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As your local and remote commands get more complex you're quickly going to become overwhelmed with trying to cram this all into one coherent script, and with hundreds or thousands of backgrounded processes you're likely to run into resource contention issues even with a beefy local machine.

You can get this under control with xargs -P. I typically break up tasks like this into two scripts.

local.sh

Generally this script has a single argument which is the hostname, and performs any necessary validations, pre-flight tasks, logging, etc. Eg:

#!/bin/bash
hostname=$1
# simple
cat remote.sh | ssh user@$hostname
# sudo the whole thing
cat remote.sh | ssh user@$hostname sudo
# log to files
cat remote.sh | ssh user@$hostname &> logs/$hostname.log
# or log to stdout with the hostname prefixed
cat remote.sh | ssh user@$hostname 2>&1 | sed "s/^/$hostname:/"

remote.sh

The script you want to run remotely, but now you don't have to cram it into a quoted one-liner and deal with quote-escaping hell.

The actual command

cat host_list.txt | xargs -P 16 -n 1 -I {} bash local.sh {}

Where:

  • -P 16 will fork up to 16 sub-processes
  • -n 1 will feed exactly one argument per command
  • -I {} will substitute the argument in place of {} [not necessary here, but may be useful for constructing more complex xargs calls.

This way even if one of your local or remote scripts gets hung up you'll still have the other 15 chugging along unimpeded.

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