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I have a script that reads from a pipe in a loop and runs an expect script and a normal shell script in the loop. Both scripts run ssh to another server to grab a piece of data. For example:

cat /tmp/file | while read a b c d do s=`expect-script server1 $b` c=`ssh $b normal-script` echo $s $c done

Even though there are many lines in /tmp/file, the script quits after processing the first line. I suspect that the expect script is swallowing all of stdin so that when it returns, there is nothing left to read. How can I avoid this? I don't want any of the scripts I call to read from the stdin of the main script.

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  • possible duplicate of "while read line" returns early when calling ssh "mysql..." Commented Jun 21, 2014 at 5:18
  • The possible duplicate is similar, but not identical. I can't use the -n option of ssh, because the ssh is called by the Expect script, which requires reading from stdin. I will play around with the redirection solution to see if that works for me. Commented Jun 21, 2014 at 5:31
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    Ah I see. Check the solution of this problem in different in another stackexchange network unix.stackexchange.com/questions/107800/… Commented Jun 21, 2014 at 5:40
  • Yes, that solution helped me get it to work, especially Richard Hansen's comment. Thank you! Commented Jun 21, 2014 at 16:59
  • @masegaloeh, I suggest adding an answer with that link, which Justin can accept, so that this question doesn't remain marked as unanswered. Commented Jun 21, 2014 at 20:56

3 Answers 3

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It's actually ssh that's slurping up stdin. Just add the -n option:

 c=$( ssh -n $b normal-script )

If you don't want to do that, you can have your shell while loop read from a different file descriptor, leaving stdin untouched:

while read -u3 a b c d do s=$( expect-script server1 $b ) c=$( ssh $b normal-script ) echo $s $c done 3< /tmp/file

(assuming bash/ksh/zsh for the read -u option)

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cat /tmp/file | while read a b c d do { s=`expect-script server1 $b` c=`ssh $b normal-script` echo $s $c } < /dev/null done

The { command... } syntax allows you to apply redirection or piping to a sequence of commands.

I'll also note that you don't need cat in your example. You could do this:

while read a b c d do ... done < /tmp/file
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Also, generally speaking, if you don't want some command to use current STDIN, you provide it with different one using <.

E.g.

 s=`expect-script server1 $b < /dev/null`

would make "expect-script" read it's STDIN from /dev/null (which happens to just return EOF) instead of consuming your current one (from /tmp/file)

You can also use | to provide some separate STDIN just for that command, e.g.

 s=`cat /tmp/stdin_for_expect | expect-script server1 $b` c=`cat /tmp/stdin_for_ssh | ssh $b normal-script`
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