mjhjmtu

I am writing a shell script that I would like to run as a daemon on startup without using external tools like daemontools or daemonize.

Linux Daemon Writing HOWTO

According to the Linux Daemon Writing HOWTO, a proper daemon has the following characteristics:

• 
  forks from the parent process


• 
  closes all file descriptors (i.e., stdin, stdout, stderr)


• opens logs for writing (if configured)


• changes the working directory to one that is persistent (usually /)


• resets the file mode mask (umask)


• creates an unique Session ID (SID)

daemonize Introduction

The daemonize Introduction goes further, stating that a typical daemon also:

• disassociates from its control terminal (if there is one) and ignores all terminal signals
  


• disassociates from its process group
  


• handles SIGCLD

How would I do all this in a sh, dash, or bash script with common Linux tools only?

The script should be able to run on as many distros as possible without additional software, although Debian is our primary focus.

NOTE: I know there are plenty of answers on the StackExchange network recommending the use of nohup or setsid, but neither of these methods tackles all of the requirements above.

EDIT: The daemon(7) manpage also gives some pointers, although there seem to be some differences between older-style SysV daemons and newer systemd ones. Since compatibility with a variety of distros is important, please ensure the answer makes clear any differences.

Using systemd you should be able to run a script as a daemon by creating a simple unit.
There are a lot of different options you can add but this is about as simple as you can get.

Say you have a script /usr/bin/mydaemon.

#!/bin/sh

while true; do
 date;
 sleep 60;
done

You create a unit /etc/systemd/system/mydaemon.service.

[Unit]
Description=My daemon

[Service]
ExecStart=/usr/bin/mydaemon
Restart=on-failure

[Install]
WantedBy=multi-user.target 

To start the demon you run

systemctl start mydaemon.service 

To start at boot you enable it

systemctl enable mydaemon.service

If on a systemd based system, which a majority of Linux distributions are today, this isn't really an external tool. The negative would be that it won't work everywhere though.

I am probably missing something here; why exactly wouldn't nohup be appropriate? Of course it's not enough alone, but supplementing it seems straightforward.

#!/bin/bash

if [ "$1" = "DAEMON" ]; then
 # is this necessary? Add other signals at will (TTIN TTOU INT STOP TSTP)
 trap '' INT
 cd /tmp
 shift
 ### daemonized section ######
 for i in $( seq 1 10 ); do
 date
 sleep 5
 done
 #### end of daemonized section ####
 exit 0
fi

export PATH=/sbin:/usr/sbin:/bin:/usr/bin:/usr/local/sbin:/usr/local/bin
umask 022
# You can add nice and ionice before nohup but they might not be installed
nohup setsid $0 DAEMON $* 2>/var/log/mydaemon.err >/var/log/mydaemon.log &

As far as I can see:

• the output is appropriately redirected (use /dev/null if necessary)


• the umask is inherited


• 
  stdin dies at the end of the parent script however


• the daemon.sh script is reparented to init (or systemd)

I have a strong feeling I'm missing the obvious. Downvote, but please tell me what it is :-)

The Linux screen command contained in most distros can daemonize a shell script. I use it often. Here's a quick example to start, list, and quit a detached screen session...

# screen -dmS Session_Name bash -c "while true; do date; sleep 60; done"

# screen -ls
There are screens on:
 8534.Session_Name (04/04/2018 08:46:27 PM) (Detached)

# screen -S Session_Name -X quit