Bash while read loop breaking early


Question

I've been banging my head against for wall for a while with this one.

I want to SSH into a set of machines and check whether they are available (accepting connections and not being used). I have created a small script, tssh, which does just that:

#!/bin/bash

host=$1
timeout=${2:-1}

ssh -qo "ConnectTimeout $timeout" $host "[ \`who | cut -f1 | wc -l \` -eq 0 ] && exit 0 || exit 1"

This script works correctly. Returning 255 if there was a connection problem, 1 if the machine is busy and 0 if everything is good. If anyone knows a better way to do this please let me know.

So next I try and call tssh on my set of machines using a while read loop, and this is where it all goes wrong. The loop exits as soon as tssh returns 0 and never completes the full set.

while read nu ; do tssh "MYBOXES$nu" ; done < <(ruby -e '(0..20).each { |i| puts i }')

At first I thought this was a subshell problem but apparently not. Any help, along with comments on style/content, would be much appreciated! I know I'm going to kick myself when I find out why...

1
28
2/23/2017 12:22:19 AM

Accepted Answer

Don't know if it would help, but a cleaner way of writing that would be

for nu in `ruby -e '(0..20).each { |i| puts i}'`; do
  tssh "MYBOXES$nu" 
done
4
12/6/2008 4:32:32 PM

Chris is correct. The source of the loop breaking was SSH using stdin, however guns is correct in is usage of a looping methodology.

If you are looping through input (a file with a list of hostnames for example), and calling SSH, you need to pass the -n parameter, otherwise your loop based on input will fail.

while read host; do
  ssh -n $host "remote command" >> output.txt
done << host_list_file.txt

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