What's the difference between nohup and ampersand (&) on Linux?

nohup and the ampersand (&) are both used to run processes in the background on Linux, but they serve different purposes and behave differently when the terminal session ends.

The Ampersand (&) Operator

The & operator runs a command in the background, allowing you to continue using the terminal while the process executes. However, the process is still attached to the terminal session.

command &

Example

sleep 100 &

[1] 12345

The output shows the job number [1] and process ID 12345. The process runs in the background, but if you close the terminal, the process will be terminated.

The nohup Command

nohup (no hang up) runs a command that ignores the HUP (hang up) signal. This means the process continues running even after the terminal session is closed. By default, nohup redirects output to a file called nohup.out.

nohup command

Example

nohup sleep 100

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

Combining nohup and &

For maximum effectiveness, combine both nohup and & to run a process in the background that survives terminal closure:

nohup command &

Example

nohup python script.py &

[1] 12346
nohup: ignoring input and appending output to 'nohup.out'

Key Differences

Common Use Cases

• Use & alone when you want to run a quick background task but don't mind if it stops when you close the terminal.

• Use nohup when you need a process to continue running after logging out, such as long-running scripts or server processes.

• Use nohup & for the best of both worlds − background execution that survives terminal closure.

Managing Background Processes

You can monitor and control background processes using these commands:

jobs          # List active jobs
fg %1         # Bring job 1 to foreground
bg %1         # Resume job 1 in background
kill %1       # Kill job 1
ps aux        # List all running processes

Conclusion

The ampersand (&) runs commands in the background but terminates them when the terminal closes, while nohup ensures processes survive terminal closure. For persistent background processes, combining both (nohup command &) provides the most reliable solution.