On walkabout in life and technology

Keep Long Running UNIX Commands Alive

Here’s the situation: you have a UNIX script that takes a long time to run, but gets killed when your computer goes to sleep and the SSH terminal or VPN disconnects. Ouch.

I know of three ways to kick long running scripts off and not worry about sleeping computers or disconnections: nohup, disown or tmux.

Simple: nohup

From the man page:

nohup -- invoke a utility immune to hangups

Which really does not tell the story. When the parent terminal session terminates, it sends a hangup (SIGHUP) to all child processes, causing them to terminate as well. nohup tells a process to ignore that hangup.

However, if the process is still part of the primary thread of a terminal, it dies anyway because it’s parent terminates.

To kick off a long running process on unix, type

$ nohup <the command> &

This tells <the command> to ignore hangups, and detaches the process from the current terminal, so the kill signal is also ignored.

To see what is going on in this thread:

$ tail -f nohup.out

nohup helpfully appends standard output and standard error to this file.

Go ahead and try it, kick off a command using nohup ... &, close the terminal, create a new terminal session and tail the nohup.out file to see that the process is still running.

Oops I Forgot: disown

If you forget to kick off a process using nohup, never fear, disown is your friend. The only negative is that this does not create an output file, so you need to use ps to track progress.

To disown a process, you need to suspend it (⌃Z), put it into the background (like the & in the nohup line) and then disown it as follows:

# press Ctrl+Z to suspend the program
$ bg
$ disown

It’s now safe to let the terminal session die, the process will run to completion.

Complex: tmux

If you already use tmux sessions, this is the simplest solution. If you do not, you need to set it up before you run the long running command.

To start a tmux terminal, just type

$ tmux new -s long-running-session

Run the program in this session as normal, no need for nohup or disown.

Before closing the terminal, detach the session:

$ tmux detach

Or ⌃b d from within the tmux terminal.

To get it back later, just re-attach the session:

$ tmux a


$ tmux a -t long-running-session

And the long running command will still be running (or completed).

I won’t go into setting up tmux, Daniel Miessler wrote a great tmux Tutorial and Primer to show you how to do this.

iTerm 2

Since I use iTerm 2 profiles to ssh to my servers (See Fast SSH Windows with iTerm 2), the simple nohup ... & version is the one I use all the time to kick off and log long-running UNIX commands.

For example, to run a long running command on Farragut, I

  • Launch iTerm 2: ⇧⌃t
  • ssh to Farragut: ⌃⌘F
  • nohup <the command> &

And if my VPN session disconnects, I can always get back an see what happened in the nohup.out file.


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