43 lines
2.5 KiB
Markdown
43 lines
2.5 KiB
Markdown
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date = "2019-08-28"
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title = "How to background a running process over SSH"
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tags = ["linux", "freebsd", "devops", "bash", "zsh"]
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description = "I know about screen, and tmux, and nohup, but sometimes I'm working and start a command that is running longer than expected. What to do?"
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Today I was logged in on my FreeNAS server to setup Duplicity backups. After initial success on a small dataset of about 100MB, I cloned my configuration to backup my long term storage dataset. It contains about 50GB of data - or so I thought. It turns out there's over 500GB there.
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I let the job run throughout the day, not minding it much. But then time comes to close my SSH connection, and I know what that means: the process will be killed. And I really **do not** want to kill this running backup.
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I know about `screen`. And `tmux`. And `nohup`. Even about `<command> &`. But alas it's too late for that.
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Luckily, there's solution to this (if you're running Bash or Zsh at least, I didn't do a compatibility chec).
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## 1. Background your running process
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Your process is probably running in the foreground, so you'll need to background it. There are two steps to this.
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1. Stop your process with `Ctrl-Z`. This basically pauzes the process, it does not kill it.
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2. Background the process by issueing the `bg` command. This will continue the process where it left off in the background. No harm done.
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## 2. Disown the process
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The final step is to use `disown` to remove the job from the current shell. This means than when the shell is killed when you sign off, the process is not killed as well.
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You can use `jobs` (or `jobs -l` on FreeBSD) to see a list of current jobs and their process ID. To disown a process, use `disown -h <process id>`.
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That's all. Your process has now been removed from your shell session and is running happily in the background.
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## Bonus action: Background your process from _another_ SSH session
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In case you can't use `Ctrl-Z`, you can log in with another SSH session and use the `kill` command to stop and continue the process that way.
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1. Sign in with another session and use `ps` to find the process ID.
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2. `kill -SIGSTOP <process id>` to stop the process
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3. `kill -SIGCONT <process id>` to continue the process in the background.
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4. Use `disown` as described above to disconnect the process from it's parent shell process.
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## Disclaimer
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This approach does not with _all_ processes, namely those that require access to the TTY to function. `vim` is a good example of this. If you background Vim, it will be stopped and you cannot disown it.
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