Create a Daemon that survives `SIGHUP` using `nohup` and `disown`

Creating a Resilient Daemon in Linux Using nohup and disown

In Linux and Unix systems, handling background processes efficiently is a crucial skill for users and administrators alike. Daemons – background processes that run independently of user sessions – can sometimes crash or terminate unexpectedly upon logout due to the SIGHUP (hangup) signal. In this article, we'll explore how to create a daemon that survives this hangup signal using the nohup and disown commands.

Q&A on Creating a Resilient Daemon

Q1: What is a SIGHUP signal, and why does it affect running processes?

A1: The SIGHUP signal is a signal sent to a process when its controlling terminal is closed. It was originally designed to notify the process of a telephone or modem hangup. In the context of Unix-like systems, when you log out of a session, the system sends SIGHUP to all the background processes initiated from that shell, signaling them to terminate because their controlling terminal will no longer exist.

Q2: What is nohup, and how does it help in preventing the process from being terminated?

A2: nohup, short for "no hangup," is a command that is used to run another command and ignores all SIGHUP signals. This utility prevents the child process from being terminated when the session is closed or when the user logs out.

Q3: What is the role of disown in managing background processes?

A3: disown is a shell builtin command that removes jobs from the current shell's job table, which means the shell no longer manages these jobs, and as a result, they are spared from SIGHUP. It's useful when you want to ensure that a background job runs even after the shell is closed.

Q4: How can I use nohup and disown together to create a daemon?

A4: To turn a process into a daemon that's resilient to SIGHUP, you can start it with nohup, run it in the background by appending & to the command, and then use disown to remove it from the shell’s jobs table. This combination ensures the process continues running irrespective of terminal hangups or user logouts.

Simple Examples and Explanation

Let's consider a simple script that continuously writes the current time into a file:

#!/bin/bash
while true
do
    echo $(date) >> /tmp/timestamps.txt
    sleep 1
done

To run this script as a daemon that survives SIGHUP:

1. Make the script executable:

   chmod +x script.sh
   

2. Start the script with nohup and send it to background:

   nohup ./script.sh &
   

3. Use disown to remove the process from the jobs list:

   disown
   

Executable Script Demonstration

Let's put our knowledge into practice with an executable demonstration:

#!/bin/bash

echo "Starting the daemon process..."
nohup bash -c 'while true; do echo $(date) >> /tmp/timestamps_daemon.txt; sleep 5; done' &
disown

echo "Daemon started. It will write timestamps every 5 seconds into /tmp/timestamps_daemon.txt."

Conclusion

By harnessing the power of nohup and disown, you can ensure that your background processes or daemons in Linux are robust against session terminations and can keep running without interruption. This approach is particularly useful in server environments and for long-running tasks essential to system operations or data processing workflows. Remember, mastering these commands can greatly enhance your ability to manage processes effectively in a Linux environment.

Further Reading

For further reading on managing background processes in Linux and utilizing nohup and disown, consider the following resources:

• Understanding Linux nohup in greater detail: https://www.geeksforgeeks.org/nohup-command-in-linux-with-examples/ This link provides a more comprehensive breakdown of the nohup command including examples.

• Guide to using Bash's job control and disown: https://linuxize.com/post/how-to-use-linux-nohup-command/ Explore job control in Bash with a specific focus on how to use disown effectively among other utilities.

• Practical uses and limitations of nohup: https://www.baeldung.com/linux/nohup-command A practical guide discussing not only how to use nohup, but also its limitations and what scenarios it is best used for.

• Detailed exploration of Unix SIGHUP: https://www.cyberciti.biz/faq/unix-linux-disown-command-examples-usage-syntax/ This article explores the concept of Unix signals, particularly focusing on SIGHUP and its impacts on processes.

• Systems Administration: Managing Background Processes and Daemons: https://opensource.com/article/17/3/introduction-linux-signals Offers insights into how system administrators can manage background processes and daemons effectively using different commands and signals across Linux systems.