While loop is causing ssh command to exit from script after first iteration.

Shell Programming and Scripting
1

I am trying to check multiple server's "uptime" in a loop over "ssh".
When I execute multiple ssh commands with hard coded servernames script is executing fine.

But when I pass server names using while loop, script is exiting after checking first server's status, why?

# serverList
serverA
serverB
serverC

# checkServerStatus.sh
#!/bin/bash

# This block got executed fine without exiting from script.
ssh serverA uptime
ssh serverB uptime
ssh serverC uptime

# This block is exiting from script after executing ssh once with serverA.
while read server; do
        ssh $server uptime
done < serverList

> 
# output 
$> bash -xv ./test.sh
#!/bin/bash

ssh serverA uptime
+ ssh serverA uptime
 04:38am  up 39 days  0:23,  1 user,  load average: 0.00, 0.02, 0.05
ssh serverB uptime
+ ssh serverB uptime
 04:38am  up 38 days 22:40,  0 users,  load average: 0.01, 0.04, 0.05
ssh serverC uptime
+ ssh serverC uptime
 04:38am  up 39 days  5:47,  0 users,  load average: 0.00, 0.01, 0.05


while read server; do
        ssh $server uptime
done <serverList
+ read server
+ ssh serverA uptime
 04:38am  up 39 days  0:23,  1 user,  load average: 0.00, 0.02, 0.05
+ read server

If I comment "ssh $server uptime" and simply print it "echo ssh $server uptime", while loop is not exiting.

How can I fix this or work around this?

2

From your description, one might guess that both read and ssh are reading text from serverList (although it isn't obvious why).

Try changing:

        ssh $server uptime

to:

        ssh $server uptime < /dev/null
3

From man ssh

while read server; do
        ssh -n $server uptime
done <serverList

That would work as long as ssh is not expecting a password or paraphrase.

Since you are using bash, you can as well change the file descriptor that the read will read from.

while read -u12 server; do
        ssh $server uptime
done 12<serverList
4

Don, usually I would be scared to try </dev/null(empty stuff), but I tried, its working,, how is it telling ssh not to use same file descriptor to read serverList?

Aia, changing file descriptor is not working for me..

#!/bin/bash
while read -u12 server; do
        ssh $server uptime
done 12 < serverList

output:
./test.sh: line 4: syntax error near unexpected token `12'

ssh -n is working for my current script. But I do see myself using ssh in a interactive manner and enter passwords to remote server login.. in that case I guess while can't be an option..

Interestingly when I replaced while with for loop, its working fine.. since my input is simple I can afford, but this is not the right way for complex inputs.

for server in $(cat $serverList); do
        ssh $server uptime
done

How can tell ssh not to read from serverList file? -u12 trick is not working on SuSE linux 11 I am using..

5

ssh - unless told otherwise - reads from stdin and empties the serverlist until EOF when used within the while loop. A for loop doesn't access stdin; there's the difference.
If you want to have interactive input as well as file input, you'll have to separate both streams, as Aia proposed using a different input file descriptor for the serverlist.

6

How can I separate file descriptor only for ssh without using "-n"?
I am using SuSE linux 11, "while read -u12 server; do" trick is not working.
I also tried "while read server <&10; do", its not working either.

---------- Post updated at 04:20 AM ---------- Previous update was at 04:13 AM ----------

I would propose this solution. Use a separate file descriptor for "while read", I picked 9 here(when for is not a choice :-).

#!/bin/bash
while read server <&9; do
        ssh $server uptime
done 9<zlist
7

The 12< trick IS working, as long as you comply the the correct syntax (no space!).
And, <&12 would duplicate stdin from fd 12 which is assumedly unassigned and thus "doesn't work". Had you assigned fd 12 to some file prior to using it, it would have worked.

8

what is the difference between these

ssh -n $server uptime

ssh $server uptime </dev/null

Found out that in bash "-u <filedescriptor>" is preferred way.. it needs space in between which I missed from Aia's post.
below code is also working now..

while read -u 9 server; do
        ssh $server uptime
done 9<serverList
9

man ssh :