permanently running script on remote machine in Unix

Question

I am having a problem working with SSH in Unix to run a script on a remote machine. The script is meant to run infinitely until an event happens, in which case it will then break out of its loop. Unfortunatly it seems that after a short peroid of afew secounds the script stops running. I believe this is related to SSH as the script when ran localy works as intended.

Is there a way I could trigger my remote machine to run that script as if it was being ran locally, so that I could close the SSH conneciton and it would still be running?

The script that calls the script located on the remote machine is this.:

 #!/usr/bin/expect set add [lindex $argv 0] set add2 [lindex $argv 1]
spawn ssh -l root2 ${add} /home/jason/localCopy/selfMonitor &
expect *assword: send "BIGMASSIVESECRET\r"
expect "]$ "

Contained in the selfMonitor script is basically a do while true statement that will run forever until broken.

Any help is appreciated, thanks.

stop using expect, and just pass the password directly to ssh? or better yet, use keys. Anyways, this feels more like a superuser.com/serverfault.com type of question. — falstro
– falstro, Commented Dec 13, 2010 at 18:23

Steve V. · Accepted Answer · 2010-12-13 18:28:03Z

1

Yes, you can do this using screen

Log in to remote machine
Start a new screen session by typing: screen
Then run your script as normal: ./myscript
Then, to detach: Ctrl-A, D

To reattach later, log in to the machine, and run screen -r to reattach to the running session.

answered Dec 13, 2010 at 18:28

Steve V.

4534 silver badges14 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

John Kugelman · Accepted Answer · 2010-12-13 18:36:02Z

1

Use nohup to have your script ignore SIGHUP, which is sent to backgrounded processes when the parent shell exits.

spawn ssh -l root2 ${add} nohup /home/jason/localCopy/selfMonitor &

Also, ssh intentionally makes it difficult to script the password entry and you are encouraged not to work around this behavior. The right way to allow password-less ssh access is to use pre-authorized keys.

Run ssh-keygen on the client. You can press enter at all the prompts to use a blank password.
Add the contents of ~/.ssh/id_rsa.pub to the end of a file called authorized_keys on the server: root2@server:~/.ssh/authorized_keys.

Creating a key and then adding it to authorized_keys allows the client to connect to the server without using a password, and it lets you do it without hard coding a password into a plaintext script.

edited Dec 13, 2010 at 18:36

answered Dec 13, 2010 at 18:29

John Kugelman

364k70 gold badges555 silver badges600 bronze badges

1 Comment

Tim Perry Over a year ago

I think this answer is spot on (+1). A couple comments, (1) I usually redirect all output to a file using >&. If you don't then nohup writes it to some default file. This gets confusing if you have two or more processes running this way. (2) I use expect, a tcl package, to enter passwords in ssh when I can't use a pre-authorized key. This is rare, but worth knowing it works.

falstro · Accepted Answer · 2010-12-13 18:31:02Z

To make your script run forever you need to do two things

Make sure you don't write to the terminal. Close it or redirect it stdout and stderr, and close stdin. If you don't they'll keep the SSH open, and / or cause your script to terminate on a broken pipe.
You need to trap SIGHUP which is sent to your process when the terminal disconnects, and normally results in the script terminating.

This is done differently in different languages, so for your language of choice, you need to look it up. In bash you use exec >logfile 2>&1 to redirect both stdout and stderr to logfile, and trap "" HUP to ignore the HUP signal. If you can, start a new process group as well, to reset the parent pid to init (officially making it a daemon process).

If you need to access the terminal output from the script (and can't redirect for whatever reason), may I recommend screen.

salva · Accepted Answer · 2010-12-13 20:21:32Z

0

Another approach is to demonize the script so that it detaches itself from the IO streams connected to SSH and starts a new session on startup.

Here there is a nice HOWTO.

And if your script is programmed in some popular language as Perl or Python, you will be able to find modules to do it.

answered Dec 13, 2010 at 20:21

salva

10.3k4 gold badges31 silver badges61 bronze badges

Comments

GrgaMrga · Accepted Answer · 2019-12-11 22:37:59Z

Although solutions here are valid, I've started using tmux extensively for such tasks.

You will be able to start a session, detach and reattach anytime you want. You can disconnect from your remote machine and it will still be running. Sessions can even have multiple windows, etc.

If you have 10 minutes, try this simple guide to tmux: A Quick and Easy Guide to tmux

After going through the guide, I've written a small cheatsheet that was in front of me for a day or two, but it's now almost a second nature to me. I've been using tmux for other tasks such as running a backend server in one pane, frontend in the other while having another pane just for usual terminal business. When you're done with working on a certain project, you can just detach and attach to another session to continue where you left off.

Collectives™ on Stack Overflow

permanently running script on remote machine in Unix

5 Answers 5

Comments

1 Comment

Comments

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

5 Answers 5

Comments

1 Comment

Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related