"Proper" way to run shell script as a daemon

Question

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.

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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)

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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

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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.

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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.

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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.

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Answer 1

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

Don't forget to sudo chmod +x /usr/bin/mydaemon.

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 daemon run

systemctl start mydaemon.service 

To start it at boot, 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 is that it won't work everywhere.

Answer 2

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 anyway
• 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 :-)

Answer 3

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