Don’t hang up!

I started using multiuser Unix systems in graduate school and I quickly learned to love 'nohup'. It let you do the impossible thing: you logged into a machine, started a long computation and then logged out! You didn’t have to hang around guarding the terminal till 2am. You could go get some sleep, hang out with friends, whatever.  This post is a short survey of ‘nohup’ like things that I’m still using and learning about, a decade and a half later.

Screen: For the longest time nohup <some command> & was my staple. I’d package all my code so that it was non-interactive, reading instructions from a parameter file, and writing out everything to data and log files, fire it off and come back the next day. Then someone introduced me to ‘screen‘. This was amazing. It let me reattach to a terminal session and carry on where I left off, allowing me to keep sessions open as I shuttle between work and home. I like doing screen -S <a name> to start sessions with distinct names.

Mosh: The one thing about screen is that I have to ssh back into the machine and start it up again. Mosh is a client server system that allow roaming, which means in practice, I can open a terminal, work on it, send my computer to sleep, work on the train with no WiFi, get into the office, and when my computer finds a WiFi again, Mosh takes up where it last left off, seamlessly – except for a little glitching on the display. Mosh doesn’t quite replace screen though – if I reboot my laptop or shutdown the terminal app, I lose the Mosh client. The session keeps running, but I can no longer interact with it.

reptyr: Ok, this is a fun one. Now suppose you have a running process that you started in a normal ssh session. Then you realize, this is actually a long running process. Oh great, the flashbacks to the late night terminal guarding sessions in grad school come back. There’s a program caller reptyr that lets you hop a running process from one terminal to another. What you want to do in this case, is start up a screen session and then invoke reptyr from the screen session.

Unfortunately I’ve not been able to get it to work properly – I always get

Unable to attach to pid 11635: Operation not permitted
The kernel denied permission while attaching. If your uid matches
the target's, check the value of /proc/sys/kernel/yama/ptrace_scope.
For more information, see /etc/sysctl.d/10-ptrace.conf

In my original terminal the program just stops and hands me back my command prompt. Nothing happens in my new terminal, but I can see that the program is running. It’s a bit like doing disown, which in turn is like doing nohup retroactively – your process can keep running, but there is no terminal. Fortunately you redirected stderr to a file by doing 2> err.txt, right? right?

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