UNIX for Dummies Questions & Answers

Hi i m writing a script which will fetch data from different machines and display it on single terminal.

I have created a file named SERVERNAMES containing ip address of machines i need to monitor.
Then for every IP i m issuing ssh command to get date on that machine.
However When i m trying it on the script it is just giving me output of first server and it is ignoring rest servers mentioned in file.

ssh works find without password for all machines mentioned in SERVERNAMES.

Code:

# cat SERVERNAMES
10.180.45.231
10.180.45.235
10.180.45.238
10.180.45.211

Script

Code:

#!/bin/bash

while read line
do
#Calculate Load on remote machine.
Date=`ssh -T -q $line date`
echo "Date on $line is:$Date"
done < SERVERNAMES

Code:

output.
Date on 10.180.8.231 is:Wed Jul 14 10:48:46 IST 2010

please help as i m not getting the output for .

Code:

10.180.45.235
10.180.45.238
10.180.45.211

Is there anything wrong in ssh?

I had the same problem, seems that it's a problem to use ssh connection in a while loop, the loop works just for the first time


using a "for" loop solved this problem for me

Code:

for line in $(cat servernames.txt)
do
Date=`ssh -T -q $line date`
echo "Date on $line is:$Date"
done

working code u just need to add -n option.

Code:

#!/bin/bash

while read line
do
#Calculate Load on remote machine.
Date=`ssh -T -q -n $line date`
echo "Date on $line is:$Date"
done < SERVERNAMES

The problem is that the loop and ssh (by default) read from the same stdin, and ssh "consumes" (eats) the input to feed it to the remote command, that would otherwise have been read by the loop. Otherwise, you wouldn't be able to run a shell on the remote system. Even when running a specific command, that command might still read from stdin, and since ssh reads from stdin you can supply data to it using the local stdin. Use ssh -n option to redirect stdin from /dev/null (prevents reading from stdin) and use ssh -T that runs the command without attempting to allocate a tty. ssh waits until all connections to stdin (from the ssh perspective) are closed. With an interactive shell the stdout of all child processes is connected to the shell, which maintains a single connection to the terminal. So when the shell terminates that means all connections to the stdin of the ssh session are closed. Without a shell all children are directly connected so the ssh session is forced to stay around.