Linux · process & service management

nohup

keep running after logout

nohup — no hang-up — starts a command that ignores the signal sent when a terminal closes. Combined with &, it lets a long job carry on after you disconnect from SSH.

nohup command & — start it, log out, and it is still running tomorrow.

Survives

Logout

and a dropped SSH connection

Output goes to

nohup.out

unless redirected

Use with

to get the prompt back

For services

systemd instead

restarts, logs, limits

At the terminal

Read the session first, then the notes: each numbered line in the output is answered by the note with the same number.

user@netflow — bashcoreutils

$ nohup ./backup.sh &

1[1] 4502

2nohup: ignoring input and appending output to 'nohup.out'

3$ exit

4$ pgrep -a backup

4502 /bin/bash ./backup.sh

$ tail -1 nohup.out

copied 1240 of 5310 files

Illustrative session · names, sizes and dates are examples, and every address is from a documentation or private range

Reading the output

What the numbered lines are telling you

Started in the background

The job number and process id. Note the id: it is how you will find the process later.

Where the output is going

With no terminal to print to, output is appended to nohup.out in the current directory.

Logging out

Normally this would end the script. Because of nohup, it keeps running.

Still there in a new session

After logging back in, the process is running and its progress can be read from nohup.out.

Variations worth knowing

The same tool, pointed at a narrower question. These four cover most of what it is reached for day to day.

nohup ./job.sh > job.log 2>&1 &

Sends both output and errors to a log file of your choosing.

tmux new -s work

A session you can detach from and return to — better for interactive work.

systemd-run --user ./job.sh

Runs the command as a temporary systemd unit, with logging.

disown -h %1

Protects a job that is already running from the hang-up signal.

In cybersecurity

A process that outlives your login is easy to forget and hard to notice.

Persistence

nohup is the simplest way to leave something running on a machine. Processes whose parent is init and that belong to a user who is not logged in are worth a look.

nohup.out collects everything

The output file grows without limit and may contain credentials the command printed. Redirect it somewhere deliberate, with sensible permissions.

It is not a service manager

Nothing restarts the process if it crashes, and nothing stops it. For anything permanent, write a systemd unit.

Filed under

Process & service management

Read alongside

jobsLists the commands this shell has running in the background or stopped, so they can be resumed.systemctlControls systemd services — start, stop, restart, enable at boot — and reports the state of each one.