Comparison of SysV initialization and launchd

If you're an old-school Unix or Linux person, you're probably also used to the SysV system initialization routine. Nowadays, it's slowly being replaced by upstart and the likes, but I'm going to skip that for now. This is meant as a guide for ye olde Unix neckbeards.

Basically, SysV init means:

• Daemons are controlled with scripts in /etc/init.d
• These scripts take a couple of standard arguments, such as start/stop/restart/reload
• There are three or four so-called runlevels, and the higher the runlevel, the more capable the system is (in other words, more daemons are started up)
• The sequence of starting the daemons (i.e. calling the init.d scripts with "start") is determined by specially named symlinks in directories named for the runlevel, for instance for runlevel 3, the directory /etc/rc3.d contains a bunch of symlinks to start the daemons appropriate for runlevel 3
• If you want to prevent a daemon from running, then you remove symlinks in the /etc/rcX.d directory. This is usually done through a command which manages this for you. Redhat uses chkconfig, Debian uses update-rc.d.
• To control specific daemons, you call the appropriate script in /etc/init.d with the start or stop parameter

OS X does this differently, and superficially, much simpler.

When the system boots, the launchd process is responsible for starting all daemons. To sum up the characteristics of this method:

• Daemons are controlled with the launchd command, there aren't any start/stop scripts
• Description of how to start a daemon is specified in a configuration file in the folder /System/Library/LaunchDaemons, and these files have the binary plist format, editable via the defaults command
• Launchd does not support dependencies, thus it's not possible to configure (for instance) starting the database daemon before the webserver.
• You can prevent daemons from running by using the launchctl command
• To control specific daemons, you use the launchctl command

There is a further distinction which I haven't talked about. OS X has the concept of user-specific daemons. These are called "launch agents" instead of daemons and they have their configuration files in /System/Library/LaunchAgents, and other locations specified in the launchd man page. Examples of these launch agents: the dock and software for your Wacom tablet.

launchctl examples

Listing present daemons

The stuff below is equivalent to listing the contents of the /etc/init.d directory on Linux.

To see what system daemons are present on the system;

 $ cd /System/Library/LaunchDaemons
 $ ls

Note that this does not tell you whether they've actually been configured to start, or not!

Overview of running daemons

The stuff below is equivalent to listing the contents of the /etc/rcX.d directories on Linux, or using the "chkconfig --list" command on RedHat.

Since launchd is more than just managing daemons, but also a replacement for init, it can show us some bookkeeping on previously run daemons. Use launchctl its 'list' parameter to list the currently running daemons, including the daemons that have already exited (i.e. configured for a one-time run).

To list all daemons:

 $ launchctl list

To show daemons, but filter out those that aren't currently running:

 $ launchctl list | grep -v "^-"

Controlling daemons

The stuff below is equivalent to using the scripts in /etc/init.d on Linux.

To temporarily stop a daemon, you use the unload parameter plus the config filename. For instance to stop the daemon that controls swapping memory to disk:

 $ sudo launchctl unload /System/Library/LaunchDaemons/com.apple.dynamic_pager.plist

If it says "nothing to unload", then the daemon wasn't configured to be started in the first place.

To temporarily start a daemon, use the load parameter:

 $ sudo launchctl unload /System/Library/LaunchDaemons/com.apple.dynamic_pager.plist

Adjusting the boot process

The stuff below is equivalent to using the chkconfig or update-rc.d commands in Linux.

To start or stop a daemon, and make sure it'll be started the next boot, use the -w flag. For example: to start the dynamic pager now, and after rebooting:

 $ sudo launchctl load -w /System/Library/LaunchDaemons/com.apple.dynamic_pager.plist

If the daemon wasn't configured to start before, you might want to pass the -F flag, which forces starting.

Configuring a daemon

The stuff below is equivalent to editing a script in /etc/init.d, or a configuration file in /etc under linux.

To configure the start/stop flags of a daemon, you edit the plist file with the defaults command. To read the current configuration:

 $ defaults read /System/Library/LaunchDaemons/com.apple.dynamic_pager.plist

To change a config file, simply use the defaults 'write' parameter. As an example, to disable Bonjour advertising on the network:

 $ sudo defaults write /System/Library/LaunchDaemons/com.apple.mDNSResponder \
     ProgramArguments -array-add "-NoMulticastAdvertisements"

Note that whenever software is updated, these changes get written over! OS X does not have a package system to my knowledge, which will save any adjusted configuration files.

Notes

Some notes:

• Launchd doesn't support dependencies, but it's possible to create special configuration files in /System/Library/LaunchDaemons which will start a daemon whenever a client tries accessing a particular network port.
• Both launchd and launchctl have a man page, and they're okay (not super, though).
• For more information on how to write the plist files, see also the man page of "launchd.plist"
• Should you want to periodically run a script, check out the tips here: http://blog.mattbrock.co.uk/2010/02/25/moving-from-cron-to-launchd-on-mac-os-x-server/