Explaining nohup in Bash

The aim of this page📝 is to explain the usage of nohup in Bash based on the particular example of starting a music player process in the background. My player is VLC and I am playing rain when working

function rain {
    nohup vlc --intf dummy --play-and-exit "/Users/admin/Music/rain30.mp3" >/dev/null 2>&1 &
}

• The function above resides in .bash_profilecontains a one-liner starting with a nohup command
• nohup is used to run a command immune to hangups in Bash.
• hangup is a type of signal in UNIX systems — it is the first signal on the long signal list
• the list of signals for BSD/MAC OS is at: https://man.freebsd.org/cgi/man.cgi?sektion=3&query=signal

signal

• signal is a communication mechanism which is passing information to a running process
• when a process receives a signal it …either terminates or does something else (changes the flow of execution, etc.) → signals are software interrupts.
• if on Mac, run man signal in the terminal, see some examples

Name          Default Action       Description
1  SIGHUP     terminate process    terminal line hangup
2  SIGINT     terminate process    interrupt program
3  SIGQUIT    create core image    quit program
4  SIGILL     create core image    illegal instruction
5  SIGTRAP    create core image    trace trap
6  SIGABRT    create core image    abort program (formerly SIGIOT)
7  SIGEMT     create core image    emulate instruction executed
8  SIGFPE     create core image    floating-point exception
9  SIGKILL    terminate process    kill program
...
17 SIGSTOP    stop process         stop (cannot be caught or ignored)
18 SIGTSTP    stop process         stop signal generated from keyboard

background vs dialog

• Look at the function again and note that — crucially — it ends with &

function rain {
    nohup vlc --intf dummy --play-and-exit "/Users/admin/Music/rain30.mp3" >/dev/null 2>&1 &
}

• Appending & at the end of a command runs it as a background process, that is it does not block your interaction with the terminal (that would be a dialog process)
• Using nohup with & allows a process to continue running in the background even after the terminal is closed.

To background an already running program you can do Ctrl-Z to suspend it + followed by bg. Note that backgrounded processes are still children processes of your terminal and will die if you close the terminal (this will send yet another signal, SIGHUP). To prevent that from happening you can run the program with nohup (a wrapper to ignore SIGHUP), or use disown if the process has already been started. Alternatively, you can use a terminal multiplexer as we will see in the next section

— https://missing.csail.mit.edu/2020/command-line/

signal  | key_shortcut
--------|---------
SIGINT  | CTRL + C
SIGTSTP | CTRL + Z
SIGINFO | CTRL + T

jobs and kill

• To suspend a background process, begin by finding the job_id with jobs

jobs
[1]+  Running                 nohup vlc --intf dummy --play-and-exit "/Users/admin/Music/rain30.mp3" > /dev/null 2>&1 &

• Then, use the kill command and specify that you want to use SIGTSTP with the job_id prefixed with %

kill -SIGTSTP %1
[1]+  Stopped                 nohup vlc --intf dummy --play-and-exit "/Users/admin/Music/rain30.mp3" > /dev/null 2>&1

• This has suspended/paused the rain in the background
• To resume the suspended job (the background rain) press bg or bg %1 (or other job_id if you have many)