In July I posted an article about sending commands remotely via ssh. This has been immensely useful, but one thing I really wanted to use it for did not work. Sending an ssh command that contained a variable, via a script for instance, would always fail for me, because, of course, the remote machine didn't know what the variable was.
Let me give an example. I have a script that creates user accounts. At the beginning of the script it asks me to supply a username, among other things, and assigns this to a variable in the script called $username. Kinda like this:
Later in the script that variable gets called to set the new user's username, and a whole bunch of other parameters. Still later in the script, I need to send a command to a remote machine via ssh, and the command I'm sending contains the $username variable:
This command would set the quota of the new user $username on the remote machine to that of the user systemsboy. But every time I've tried to include this command in the script, it fails, which, if you think about it, makes a whole lot of sense. See, 'cause the remote machine doesn't know squat about my script, and when that command gets to the remote machine, the remote machine has no idea who in the hell $username is. The remote machine reads $username literally, and the command fails.
The solution to this is probably obvious to hard-core scripters, but it took me a bit of thinkin' to figure it out. The solution is to create a new variable that is comprised of the ssh command calling the $username variable, and then call the new variable (the entire command) in the script. Which looks a little something like this:
So we've created a variable, called $quota, which is the entire ssh command, and then we've simply called that variable in the script. That $quota variable will have the $username variable already filled in, and the command will now succeed on the remote machine. One thing that's important to note here: generally the command being sent over ssh is enclosed in single-quotes. In this instance, however, it must be enclosed in double-quotes for the command to work. I also used the -t option in this example (which tells ssh that the session is interactive, and to wait until it's told to return to the local machine) but I don't actually think it's necessary in this case. Still, it shouldn't hurt to have it there, just in case something goes funky.
But so far nothing has gone funky. This seems to work great.