An update to the previous comment. It is true that typing `nohup emacs &` directly into the shell does not help, i.e., Emacs dies when the shell dies. Example:

  $ cat /etc/debian_version 
  10.0

  $ $SHELL --version | head -n 1
  GNU bash, version 5.0.3(1)-release (x86_64-pc-linux-gnu)
  
  $ nohup emacs &

  $ ps -ef | grep emacs | grep -v grep
  susam    22210 22046  2 13:59 pts/0    00:00:00 emacs

  $ ps -ef | grep 22046 | grep -v grep
  susam    22046 22042  0 13:45 pts/0    00:00:00 bash
  susam    22210 22046  0 13:59 pts/0    00:00:00 emacs
  susam    22224 22046  0 14:00 pts/0    00:00:00 ps -ef

The parent of the emacs process is still bash, so when bash dies, Emacs dies too.

However, invoking `nohup emacs &` from another shell script does work. The PPID of Emacs gets assigned to 1 and Emacs continues to keep running even after killing the shell. Example:

  $ bash -c 'nohup emacs &'

  $ ps -ef | grep emacs | grep -v grep
  susam    22229     1  2 14:02 pts/0    00:00:00 emacs

  $ dash -c 'nohup emacs &'

  $ ps -ef | grep emacs | grep -v grep
  susam    22229     1  0 14:02 pts/0    00:00:00 emacs
  susam    22238     1  2 14:03 pts/0    00:00:00 emacs

  $ ps -ef | head -n 2
  UID        PID  PPID  C STIME TTY          TIME CMD
  root         1     0  0 Dec29 ?        00:00:01 /sbin/init

In the second experiment, the parent of the emacs process is the init process, to it continues to run successfully even after the shell or terminal exits.

To summarize, yes, we can use nohup in the startup script and make it POSIX compliant.