For long(er) running jobs it’s useful to run jobs in the background. However, when a job is simply started in the background

> sleep 20 &

it becomes a child process of the current shell (see the PPID entry in the output from ps below; irrelevant lines removed).

> ps xlu
#   PID  PPID CPU PRI NI      VSZ    RSS WCHAN  STAT   TT       TIME COMMAND          USER    %CPU %MEM STARTED
# 34472 34320   0  31  0  2432748    540 -      S    s003    0:00.00 sleep 20         wresch   0.0  0.0  2:26PM

What happens to this child process when the current shell exits depends on the shell option huponexit

> shopt | grep huponexit
# huponexit          off

which seems to default to off on all the machines i checked. This means that when the parent shell is terminated, it does not send SIGHUP to it’s children and therefore the children continue to run. If, however, huponexit is on, children will receive SIGHUP when the parent shell is terminated and will therefore terminate themselves. This can be avoided with the nohup utility. nohup will connect stdin to /dev/null and redirect stdout and stderr to nohup.out and ignore SIGHUP. In addition, the BSD nohup on OS X apparently also disconnects the process from the controlling terminal because the parent PID is now init (see below). That does not seem to be the case for GNU nohup, which maintains the process as a child of the current terminal.

> nohup sleep 30 &
> ps xlu
#   PID  PPID CPU PRI NI      VSZ    RSS WCHAN  STAT   TT       TIME COMMAND          USER    %CPU %MEM STARTED
# 35472     1   0  31  0  2432748    540 -      S      ??    0:00.00 sleep 30         wresch   0.0  0.0  2:44PM

A way to disconnect a process from the current shell is to start it in the background in a subshell. This will make the process a child of PID 1 (init) on OS X (not shown) and Linux (shown below) - the child process was disowned.

> (sleep 20 &)
> ps xl
# F   UID    PID   PPID PRI  NI    VSZ   RSS WCHAN  STAT TTY        TIME COMMAND
# 0 33634  18796      1  20   0 100908   600 hrtime S    pts/297    0:00 sleep 20

Since the process is now not a child of the shell, it will not be sent SIGHUP when the shell terminates and therefore continues to run after terminating the current shell, but for different reasons nohup-ed processes do.