4.2 An Excursion into Daemon Management
| A daemon is a program that runs in the background (that is, without interacting with a user) and is useful. On Unix systems, there are two kinds of daemons: the ones that run continuously and the ones that run on demand. Familiar Unix examples of continuous ones include named, the DNS server, and httpd, the Apache web server, while on-demand ones include servers for network services, such as telnet and ftp, and cleanup scripts run once a day or once a week. The on-demand ones are all started from continuous servers such as cron for time-based services, and inetd or tcpserver (the qmail replacement for inetd) for network services. The daemontools package provides a consistent way to run continuous daemons, optionally (but almost invariably) also arranging to collect log information that the daemons produce. The two key programs are supervise, which controls a single daemon, and svscan, a "superdaemon," which controls multiple copies of supervise and connects each daemon with its logger. For each daemon to be controlled, supervise uses a directory containing information about the daemon. The only file that you must create in that directory is run, the program to run. Although it can be a link to the daemon, it's usually a short shell script that sets up the environment and then exec's the daemon. The supervise program creates a subdirectory also called supervise, where it stores info about what it's doing. Once supervise is running, you can use the svc program to stop and start the daemon, and send signals to it. (This consistent way to signal daemons is one of supervise's greatest strengths.) To run supervise, follow these two steps: create /service, which you do with a regular mkdir command as the super-user, and start svscan, which I cover in the next section. Once svscan is running, it looks at /service and starts a supervise process in each of its subdirectories. Every five seconds it looks again and creates new processes for any new subdirectories. If a subdirectory has a sub-subdirectory called log, svscan arranges to log the output of the program. In this case, it starts a pair of processes connected by a pipe, equivalent to: supervise subdir | supervise subdir/log The log subdirectory contains a run file that invariably runs multilog to write the output into a rotating set of log files. 4.2.1 Starting a DaemonOne of the least standardized aspects of Unix and Unix-like systems is the way that you start daemons at system boot time. Even if you use supervise as I recommend, you still must start the svscan daemon to get everything else going. Here are some hints to start svscan. If you ignore my advice and run daemons directly, start each of them the way I recommend you start svscan. Versions 0.75 and later of daemontools include a startup script for svscan called svscanboot, and the daemontools installation process tries, usually successfully, to edit a call to that program into your system startup scripts. It sets up the environment and runs svscan, piping its output into a new program called readproctitle that copies anything it reads on top of its program arguments, which means that any error messages from svscan will show up in ps listings in the arguments to readproctitle. This kludge makes it possible to see what's wrong if svscan has trouble starting up or starting supervise for any of the directories under /service:
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