For Loop for Remote SSH Command Execution

This is something new I'm trying. Instead of trying to only post about large projects, difficult tasks, or troubleshooting (which often means I have a lot of posts that are sitting around waiting to be completed), I'm going to also start including quick snippets of knowledge that I would otherwise deem to small to dedicate to a whole post. So, without further ado... 

We're working on rolling out a CMS (Config Management Service) called Salt Stack. Our work with Salt is one of those aforementioned posts that sits in editing limbo. I needed to restart the salt minion on several servers because the hostname of the Salt master had changed. I knew I could do this with a simple for loop. I created a file called servers.txt, and added all of the servers I needed to reach into it. I then called the file with this command:

for $i in `cat servers.txt`; do ssh user@$i service salt-minion restart; done

I tested this first by having the remote command be "date" instead to confirm that I had the right syntax. It worked, so I moved on to actually restarting the service. The first box worked fine:

user@server1's password:
Stopping salt-minion daemon: [  OK  ]
Starting salt-minion daemon: [  OK  ]
^CKilled by signal 2. 

It would stop and start the daemon, but not complete and move on to the next host in the list. I had to Ctrl+C to get it to ssh to the next box.

I found that I needed to use "-t" with the ssh command.

for $i in `cat servers.txt`; do ssh -t user@$i service salt-minion restart; done

What does the -t flag do? Apparently, when you log in with ssh it uses psuedo-tty. This is not the case if you use ssh to send a remote command. If you execute a remote command that only has plaintext output (like the "date" command), no problem. However, if you use -t it forces the allocation of a psuedo tty session.

Lesson learned.