Bash

Check if variable is set link

source: where ${var+x} is a parameter expansion which evaluates to nothing if var is unset, and substitutes the string x otherwise.

  if [ -z ${var+x} ]; then echo "var is unset"; else echo "var is set to '$var'"; fi
  

Looping shell with ssh link

This requires using -n because ssh otherwise will consume the while loop

  while read i; do 
  echo $i; ssh -n $i "echo $(whoami)"; 
done < /servers
  

Run a command with a specific UID link

Whether that user exists or not. Great for permission issues. Source

  sudo -u '#1234' vi file