in my bash loop over a list of some servers, if the ssh connects the bash script exits

Question

I have a quick script to run a command on each server using ssh (i am sure there are lots of better ways to do this, but it was intended to just work quick!!). For the test1 etc, there is no server so the script continues, the script also continues if the pubkey auth fails. However if the script connects, the date is printed but the ssh loop terminates...

#!/bin/bash -x

cat <<EOF |
##file servers
test1
test2
server1
server2
EOF
while read line
do
if [ "${line:0:1}" != "#"  ]; then

ssh -q -oPasswordAuthentication=no -i id_dsa user1@${line} date

fi
done

echo read line must have exited

output is like so;

+ cat
+ read line
+ '[' t '!=' '#' ']'
+ ssh -q -oPasswordAuthentication=no -i id_dsa user1@test1 date
+ read line
+ '[' t '!=' '#' ']'
+ ssh -q -oPasswordAuthentication=no -i id_dsa user1@test2 date
+ read line1
+ '[' s '!=' '#' ']'
+ ssh -q -oPasswordAuthentication=no -i id_dsa user1@server1 date
Fri Jul  9 09:04:16 PDT 2010
+ read line
+ echo read line must have exited
read line must have exited`enter code here`

something to do with the successful return of the ssh command is messing with the condition for the loop or the var... any suggestions on why?

Answer 1

I think the reason is that as ssh is being forked and exec'd in your bash script the script's standard input is being reopened so your read simultaneously terminates. Try re-crafting as follows:

for line in test1 test2 server1 server2
do
    if [ "${line:0:1}" != "#"  ]; then
        ssh -q -oPasswordAuthentication=no -i id_dsa user1@${line} date
    fi
done

or maybe run the ssh in a sub-shell like this:

( ssh -q -oPasswordAuthentication=no -i id_dsa user1@${line} date )

Answer 2

You should pass the -n flag to ssh, to prevent it messing with stdin:

ssh -n -q -oPasswordAuthentication=no -i id_dsa user1@${line} date

I tested this with my own server and reproduced the problem, adding -n solves it. As the ssh man page says:

Redirects stdin from /dev/null (actually, prevents reading from stdin)

In your example, ssh must have read from stdin, which messes up your read in the loop.