sequential SSH command execution not working in Ubuntu/Bash

Question

My requirement is I will have a set of commands that needs to be executed in a text file. My Shell script has to read each command, execute and store the results in a separate file.

Here is the snippet which does the above requirement.

while read command
do
    echo 'Command :' $command >> "$OUTPUT_FILE"
    redirect_pos=`expr index "$command" '>>'`       
    if [ `expr index "$command" '>>'` != 0 ];then
        redirect_fn "$redirect_pos" "$command";        
    else  
        $command 
        state=$?
        if [ $state != 0 ];then
                echo "command failed." >> "$OUTPUT_FILE"
        else
        echo "executed successfully." >> "$OUTPUT_FILE"
        fi
     fi
    echo  >> "$OUTPUT_FILE"
done < "$INPUT_FILE"

Sample Commands.txt will be like this ...

tar -rvf /var/tmp/logs.tar -C /var/tmp/ Commands_log.txt
gzip /var/tmp/logs.tar
rm -f /var/tmp/list.txt

This is working fine for commands which needs to be executed in local machine. But When I am trying to execute the following ssh commands only the 1st command getting executed.

Here are the some of the ssh commands added in my text file.

ssh uname@hostname1 tar -rvf /var/tmp/logs.tar -C /var/tmp/ Commands_log.txt
ssh uname@hostname2 gzip /var/tmp/logs.tar
ssh .. etc

When I am executing this in cli it is working fine. Could anybody help me in this?

Answer 1

I'm not sure I get your problem correctly. But just generate the script on the local side, copy it via scp and then execute it with ssh should be much easier.

If you want to execute it soley with ssh you should use this syntax:

ssh localhost -C "echo test; echo blub"

EDIT: Okay I finally found the problem in your script. $command is running your "command" inside the "while read" loop, and while local commands like "echo" or "tar -f" (the -f is important as it take a FILE and no input from stdin) everything works like a charm.

SSH on the other hand tries to create an "interactive" session where your keyboard input is forwarded to the other machine. The problem now is that it reads all the "waiting data" from your "< $INPUT_FILE" redirection and is sending it to the ssh host.

You can actually see this behavior. Try adding a command like:

ssh user@host tee flush.txt

Tee is now redirecting the "rest" of your $INPUT_FILE to the remote hosts file "flush.txt". So the next time we pass "done" there is nothing left to consume for "read".

To fix this behaviour you simply have to tell $command to get the input from something like /dev/null.

$command < /dev/null

will fix your issue.