How to pass the output of a script as an argument to a ssh command?

Question

I have a script that shows all the instances on AWS that I'm running:

$/home/scripts/show_instances.sh
i-f66d2de0  running 10.148.17.40
i-fxxdbbe0  running 10.148.20.60
(...)

What I'd like to do is to take the first IP, pipe to ssh and connect to the server automatically. I tried this:

$/home/scripts/show_instances.sh | tail -1 | awk '{print $3}' | xargs ssh

But is not working: it connects to the server but after the first command that I type (i.e. ls), the terminal just seems to be waiting for something and nothing happens (I'm on mac connecting to Unix), so I have to exit the instance with ctrl+c. Some suggestions?

Not sure that xargs is buying you anything in this case. If your headline is really your Q, then ssh $(..../show_instances.sh | awk 'NR==1{print $3}') ..other ssh stuff ... (using cmd-substitution $(...)) should solve your problem. (don't need tail). Otherwise it seems that your Q may be about "how do I use xargs). Good luck. — shellter
– shellter, Commented Oct 3, 2016 at 13:37
Generally you can use backticks (or $( ) ) for obtaining this (I can't show you how backticks work because they are used for formatting purposes on StackOverflow website). — Dominique
– Dominique, Commented Oct 3, 2016 at 13:42
Wow! Genius! It's working now! I did like this: ssh /home/scripts/show_instances.sh | tail -1 | awk '{print $3}' Thank you! — Arthur Accioly
– Arthur Accioly, Commented Oct 3, 2016 at 13:56
Explaining better, both ways work, like you said. ssh (backticks)/home/scripts/show_instances.sh | tail -1 | awk '{print $3}'(backticks) or using $(..) ssh "$(/home/scripts/show_instances.sh | tail -1 | awk '{print $3}')" Thank you! — Arthur Accioly
– Arthur Accioly, Commented Oct 3, 2016 at 14:24
Join the early 90's and use $(... cmds sub ) ;-) .. Backticks are your backup, if you get stuck using an ancient system that doesn't support $(..). Also, read about the awk internal variable NR, you don't need tail for this case. Good luck. — shellter
– shellter, Commented Oct 3, 2016 at 15:19

heemayl · Accepted Answer · 2016-10-03 15:11:53Z

4

Use a command substitution, and run one go of awk in that to get the desired IP from first line, and then do ssh:

ssh "$(/home/scripts/show_instances.sh | awk 'NR==1{print $NF; exit}')"

If you want to stick to the piping, and xargs one, then allocating pseudo TTY (-t) should let you in and run interactively:

/home/scripts/show_instances.sh | awk 'NR==1{print $NF; exit}' | xargs ssh -tt

For last line, do:

ssh "$(/home/scripts/show_instances.sh | awk 'END{print $NF}')"
/home/scripts/show_instances.sh | awk 'END{print $NF}' | xargs ssh -tt

edited Oct 3, 2016 at 15:11

answered Oct 3, 2016 at 13:38

heemayl

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4 Comments

Arthur Accioly Over a year ago

Worked! I did like this: ssh "$(/home/scripts/show_instances.sh | tail -1 | awk '{print $3}')" Thank you!

heemayl Over a year ago

@ArthurAccioly Check my edits. You don't need tail, you can do this with awk alone.

Arthur Accioly Over a year ago

Wow! Even better! Like this right? ssh "$(/home/scripts/show_instances.sh | awk 'END{print $NF}')" I'm not very good on awk. Why NF gets exactly the IP? btw, the second option with ssh -tt didn't work and I had the same issue of terminal waiting for something after any command, like "ls". But, anyway, I'm very happy with the command.

heemayl Over a year ago

@ArthurAccioly NF gets us the number of fields, and $NF gets the last field's value expectedly.

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