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saved by17 people, first byJoel Liu on 2007-06-18, last byKrishnan Subramanian on 2008-06-14

• 6. Add Your Public Key to Remote Machines the Easy Way
  In
  order to perform key based logins to a new machine, you need to get a copy of a
  public key to the remote machine yourself. Sure, you could do this manually –
  which gets a bit boring after a while (why doesn’t SSH have an authorized_keys.d
  anyway?), but why waste time when SSH comes with it the tool to do it?
  Just
  run:

  
  

  
  

  ssh-copy-id -i .ssh/id_rsa.pub hostname

  
  

  After being prompted to enter your password for the last time, SSH will
  say:
  Now try logging into the machine, with “ssh ‘hostname’”, and check
  in:

  
  

  .ssh/authorized_keys

  
  

  to make sure we haven’t added extra keys that you weren’t
  expecting.
  

  
  

  Try it. No more passwords!

• 2. Parallelize Your Loops
  Almost every Linux network
  administrator knows the power of the for loop: the way to do
  something for one, one hundred or one thousand users, files, machines,
  processes, or whatever else. Most people set their loops in sequence – so that
  each jobs is finished before moving onto the next.
  But the
  jobs command can be used to background each loop, so you don’t
  have to wait for it to complete before continuing with the next.

  
  

  Here’s an example running an apt-get update:

  
  

  
  

  for HOST in $(< ListOfHosts); do ssh $HOST ’sudo apt-get update’ &
  done

  
  

  Maybe you need a bunch of SSH tunnels running simultaneously:

  
  

  
  

  for HOST in $(< ListOfHosts); do ssh -C -N -R 80:localhost:80 $HOST &
  done

  
  

  Sometimes you probably don’t want to see all the output as it happens – in
  that case, save a file on each machine and use another loop to collect it
  later.

  
  

  Benefit: Saving a metric shitload (2/3rd of an imperial
  shitload) of time waiting on stuff to finish
  Works in:
  any currently supported Linux
  Drawbacks: Bash
  probably has some limits of the amount of concurrent jobs. But I’ve yet to run
  into them.