Let’s continue our discussion of Linux processes with a look at two unrelated concepts that are both good to understand. First we’ll write a simple daemon process and send signals to it. Then we’ll see how to suspend a child shell — which gives some insight into how shells cope with signals sent to them.
What’s a daemon?
A daemon is typically a long-running job that doesn’t have a controlling terminal (tty). It’s often started when the system boots — for instance, from one of the files in /etc/rc* — and it runs until shut down. It could also be run from cron or on demand.
The Linux Daemon Writing HOWTO has code examples and descriptions for GCC. The Unix Programming FAQ has a useful section titled “How do I get my program to act like a daemon?” An easy way to do the most important parts of the job is with the daemon system call. It detaches the process from its parent, redirects all standard I/O to /dev/null, and changes the current directory to the root, /.
Many daemons have a configuration file that’s read when the program starts and again if the daemon catches a signal — often SIGHUP (signal 1). This lets you update a daemon while it keeps running.
Daemon in a shell script
One way to make simple daemons is by writing a daemon-like shell script. To simplify, our sample script will send messages to its standard output. (If it were a…
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