Mac OS X Tiger for Unix Geeks, 3rd Edition

Chapter 4. Startup

The most striking difference between Mac OS X and other flavors of Unix is in how Mac OS X handles the boot process. Gone are /etc/inittab, /etc/init.d, and /etc/rc.local from traditional Unix systems. In their place is a BSD-like startup sequence sandwiched between a Mach^[*] foundation and the Aqua user interface.

This chapter describes Mac OS X Tiger’s startup sequence, beginning with the BootX loader and progressing to full multiuser mode, at which time the system is ready to accept logins from normal users. The chapter also covers custom startup items, network interface configuration, and Mac OS X’s default periodic jobs.

Booting Mac OS X

When the computer is powered up, the firmware is in complete control. After the firmware initializes the hardware, it hands off control to the BootX loader, which bootstraps the kernel. After a trip into Mach, the control bubbles up into the BSD subsystem, and eventually into the Aqua user interface.

By default, Mac OS X boots graphically. If you’d like to see console messages as you boot, hold down ⌘-V (the “V” stands for “verbose”) as you start the computer. If you’d like to always boot in verbose mode , you can specify a flag in the boot arguments that are stored in your system’s firmware. First, use the command nvram boot-args to make sure there aren’t any flags already set (if there are, and you didn’t set them, you probably should not change this setting). Set your boot arguments to -v with this command:

    sudo /usr/sbin/nvram boot-args="-v"

The next time you boot your Mac, it boots in verbose mode. To turn this setting off, use the command:

    sudo /usr/sbin/nvram boot-args=

To boot in single-user mode , hold down ⌘-S as you start the computer. In single-user mode, your filesystem is mounted as read-only, which limits what you can do. Single-user mode should generally be used only to repair a system that has been damaged (for example, see "Restoring the Directory Services Database" in Chapter 5). Unlike with other Unix systems, we do not suggest that you use single-user mode to perform fsck repairs manually. Instead, restart your Mac and boot from Tiger’s install DVD (hold down the C key as your Mac starts up), and then run the Disk Utility (Installer → Open Disk Utility) to repair a problem disk volume.

The BootX Loader

BootX is located in /System/Library/CoreServices. It draws the Apple logo on the screen and proceeds to set up the kernel environment. BootX first looks for kernel extensions (drivers , also known as kexts) that are cached in the mkext cache. If this cache does not exist, BootX loads only those extensions in /System/Library/Extensions that have the OSBundleRequired key in their ExtensionName .kext/ Info.plist file. Example 4-1 is an excerpt from the /System/Library/Extensions/System.kext/Info.plist file.

Example 4-1. A portion of a kernel extension’s Info.plist file

    <?xml version="1.0" encoding="UTF-8"?>
    <!DOCTYPE plist PUBLIC "-//Apple Computer//DTD PLIST 1.0//EN"
              "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
    <plist version="1.0">
      <dict>
        <key>CFBundleDevelopmentRegion</key>
        <string>English</string>
        <!-- multiple keys and strings omitted -->
      </dict>
    </plist>

After the required drivers are loaded, BootX hands off control to the kernel (/mach_kernel).

Initialization

The kernel first initializes all the data structures needed to support Mach and BSD. Next, it initializes the I/O Kit, which connects the kernel with the set of extensions that correspond to the machine’s hardware configuration. Then, the kernel finds and mounts the root filesystem. The kernel then launches the first process on the system, launchd , which is responsible for bootstrapping the system as well as launching daemons on behalf of the system or users.

Tip

Mac OS X Panther (10.3) and earlier did things differently. The first process the kernel loaded was mach_init, which started Mach message handling. mach_init then launched the BSD init process. In keeping with Unix conventions, init was process ID (PID) 1, even though it was started second. mach_init was given PID 2, and its parent PID was set to 1 (init’s PID). As of Mac OS X Tiger (10.4), launchd replaces both of these processes.

The /etc/rc Script

If you’re booting normally, launchd loads the /etc/rc shell script to start the system (if you’re booting in single user mode, /etc/rc is not run at all). This rc script sources the /etc/rc.common script, which sets the initial environment, defines some common functions, and loads the /etc/hostconfig file, which controls the system services that get started at boot. Example 4-2 is an excerpt from the hostconfig file.

Example 4-2. A portion of /etc/hostconfig

    # Services
    AFPSERVER=-NO-
    CUPS=-AUTOMATIC-

This excerpt shows the settings that determine whether Apple File Sharing and CUPS (Common Unix Printing System) are launched at startup. See "The startup script" later in this chapter, for an explanation of how /etc/hostconfig can be used to control services that you install yourself. Table 4-1 describes the default entries from /etc/hostconfig.

Table 4-1. Default entries from the hostconfig file

Entry	Default value	Description
HOSTNAME	-AUTOMATIC-	Specifies a hostname. A setting of `-AUTOMATIC-` causes configd to use the value from the system configuration database.
ROUTER	-AUTOMATIC-	This specifies the default router. `-AUTOMATIC-` causes Mac OS X to use the router supplied by DHCP or BOOTP. The settings in System Preferences → Network override this.
AFPSERVER	-NO-	Controls whether Apple File Sharing (Personal File Sharing in System Preferences → Sharing) is enabled. This corresponds to the AppleShare startup item. (For information on startup items, see "SystemStarter,” later in this chapter.)
AUTHSERVER	-NO-	Specifies whether the NetInfo authentication server for legacy clients (/usr/sbin/tim) should be started. This corresponds to the AuthServer startup item.
AUTOMOUNT	-YES-	Determines whether the NFS automount daemon should be started. The NFS startup item consults this setting.
CUPS	-AUTOMATIC-	Controls whether Printing Services are started up. This corresponds to the PrintingServices startup item. However, this is not controlled by the Printer Sharing option in System Preferences → Sharing (that setting instead inserts the appropriate settings into the /etc/cups/cupsd.conf file).
IPFORWARDING	-NO-	Determines whether the Network startup item enables IP forwarding.
IPV6	-YES-	Specifies whether the Network startup item should turn on IPv6 support.
NFSLOCKS	-AUTOMATIC-	If your Mac is running as an NFS server, a setting of `-AUTOMATIC-` enables locking for NFS files. As an NFS client, a value of `-YES-` enables locking, but `-AUTOMATIC-` loads the appropriate daemons (rpc.statd and rpc.lockd) so they are only used when needed. The NFS startup item consults this setting.
NISDOMAIN	-NO-	Specifies the NIS Domain that your Mac should participate in. Leave it set to `-NO-` to disable NIS, otherwise set it to the appropriate domain. The NIS startup item uses this setting.
TIMESYNC	-YES-	Controls whether the network time daemon (ntpd) is started. You can configure these settings with System Preferences → Date & Time. This setting affects the NetworkTime startup item.
QTSSERVER	-NO-	Specifies whether the QuickTime Streaming Server is started at boot time. Although it’s not included with the desktop version of Mac OS X, you can download it from http://developer.apple.com/darwin/projects/streaming/.
WEBSERVER	-NO-	Controls whether the Apache web server (Personal Web Sharing in System Preferences → Sharing) is started. This corresponds to the Apache startup item.
SMBSERVER	-NO-	This setting has no effect. Older versions of Mac OS X used it to control Samba , the Windows file sharing server. This setting can be toggled using Windows Sharing in System Preferences → Sharing, which toggles the `disable` setting in /etc/xinetd.d/smbd.
SNMPSERVER	-NO-	Specifies whether the SNMP agent is to be started. This corresponds to the SNMP startup item.
CRASHREPORTER	-YES-	Controls whether the crash reporter is started. You can refine the crash reporter settings with /Developer/Utilities/CrashReporterPrefs.
ARDAGENT	-NO-	Specifies whether Apple Remote Desktop (also specified in System Preferences → Sharing) is started.

After rc has loaded in values from /etc/rc.common and /etc/hostconfig, it performs a check of the filesystem (fsck) if needed. If the fsck fails, rc tries an fsck -fy, which forces a check and assumes a “Yes” answer to all the questions that fsck asks. If that fails, the system halts.

Tip

If you find yourself with fsck problems, you should boot from the Mac OS X installation DVD. You can boot from a DVD by holding down the C key at startup. When the Installer appears, choose Installer → Disk Utility from the menu bar and use it to inspect and repair the damaged disk.

Next, /etc/rc performs the following steps (among others, that is; this list describes the most significant):

Mounts local filesystems: By this point, the root filesystem is already mounted, but the rc script now mounts any additional HFS+ and UFS volumes listed in /etc/fstab, as well as the /dev filesystem. This step does not, however, perform the automatic mounting of local volumes under the /Volumes directory. This is handled by the disk arbitration daemon, which is started as a Mach bootstrap daemon (see "Mach Bootstrap Services,” later in this chapter).
Launches BootCacheControl: The rc script initializes the boot-time performance cache (BootCacheControl), which implements intelligent read-ahead strategies for the boot volume.
Tunes the system: Next, a series of sysctl calls tune kernel variables such as the maximum number of vnodes (data structures the kernel uses to represent files) and various shared memory settings.
Enables virtual memory: At this point, the dynamic_pager daemon starts running. This daemon manages swap files in the /var/vm/ subdirectory. The kernel uses these files to allocate virtual memory as it is needed.
Starts kextd, the kernel extension daemon: The kernel initially boots with the minimum set of extensions needed to mount the root filesystem on all supported hardware. Some of these extensions are not needed, so /etc/rc starts the kextd daemon (/usr/libexec/kextd) to unload unnecessary extensions. For example, the iPodDriver includes the OSBundleRequired key to support booting from your iPod. If you don’t have your iPod plugged in, kextd can safely unload that driver. The kextd daemon is also responsible for loading and unloading extensions on demand for the duration of the system’s uptime. Extensions live in the /System/Library/Extensions directory.
Creates the NetInfo Database: NetInfo is a Directory Services database for standalone machines. See Chapter 3 for a complete discussion. In this step, the rc script creates a default NetInfo database (only if none exists).
Launches Mach bootstrap services: Next, the rc script runs register_mach_bootstrap_servers on all the services listed in /etc/mach_init.d. That directory contains a collection of XML .plist files containing a description of services, the path to the corresponding executable, and whether the service should be loaded on demand. See "Mach Bootstrap Services,” later in this chapter.
Starts up Launch Daemons: As of Mac OS X 10.4 Tiger, the SystemStarter (see "SystemStarter,” later in this chapter) method for starting up daemons is reserved for core system components such as Apache, Spotlight, and Apple Remote Desktop. In its place, launchd, which is also the first process on the system, takes care of bootstrapping most of the services on the system. See "launchd,” later in this chapter.
Sets the system language: If this system is not fully configured (if the file /var/db/.AppleSetupDone does not exist), the language chooser appears at this point and prompts the user to choose a default language for the system. Whether that chooser appears, the rc script reads in /var/log/CDIS.custom and exports the variable it contains into subsequent environments.

After these steps are completed, /etc/rc hands off control to /sbin/SystemStarter, which is described in the "SystemStarter" section later in this chapter.

Tip

It’s true—there are three ways to start a daemon on Mac OS X: using SystemStarter, Mach Bootstrap Services, and launchd. Of the three of these, launchd is the latest, but for some purposes, you’ll find the SystemStarter scheme to be best. For more information, see "SystemStarter" and "Adding Startup Items,” later in this chapter.

Mach Bootstrap Services

Mac OS X Panther introduced Mach bootstrap services, a new approach for starting daemons. Daemons can be loaded at two points: system startup and user login, which includes local and remote (such as SSH) logins. System startup scripts go into /etc/mach_init.d, and user login scripts go into /etc/mach_init_per_user.d. Bootstrap daemons are identified to the system using the ServiceName in their .plist files, and the operating system can load that service on demand if the OnDemand option is set to true (this is the default). Mac OS X will launch these services on demand or wake sleeping bootstrap services (when a bootstrap service goes unused for a period of time, it can sleep). Table 4-2 describes the services started in this stage.

Table 4-2. Mach bootstrap services

Item	Description
ATSServer.plist	Launches the Apple Type Solution server.
configd.plist	Starts the Configuration server daemon. See "scutil" in Chapter 17 for information on working with the Configuration server’s database.
coreaudiod.plist	Starts the Core Audio daemon.
coreservicesd.plist	Launches the Core Services daemon.
DirectoryService.plist	Starts The DirectoryService daemon. For more information, see Chapter 5 and the DirectoryService manpage.
diskarbitrationd.plist	Launches the disk arbitration daemon, which coordinates the mounting of filesystems. For more information, see the diskarbitrationd manpage.
distnoted.plist	Starts the distributed notifications daemon.
hdiejectd.plist	Unknown or undocumented.
IIDCAssistant.plist	Starts a daemon required to support iSight audio.
KerberosAutoConfig.plist	Configures the single sign-on service. See the kerberosautoconfig manpage .
kuncd.plist	Starts the Kernel-User Notification daemon, which kernel-level code can use to pop up dialogs when user action is needed. See the “Kernel-User Notification” topic in Writing an I/O Kit Device Driver, which you can find at http://developer.apple.com/documentation/DeviceDrivers/.
lookupd.plist	Starts lookupd, a thin layer that acts as a frontend to Directory Services. For more information, see the lookupd manpage and Chapter 5.
mds.plist	Starts the mds daemon used by Spotlight.
memberd.plist	Starts the group membership daemon.
notifyd.plist	Launches the server for the Mac OS X notification system. For more information, see the notify(3) manpage.
ocspd.plist	Starts the Online Certificate Status Protocol daemon, used to check the status of a certificate.
scsid.plist	Launches a daemon used by the SCSI subsystem.
securityd.plist	Starts the security server, which manages keychain items and other cryptographic operations.
WindowServer.plist	Starts the Mac OS X WindowServer, the service that manages the screen and the windows drawn upon it.

launchd

Mac OS X Tiger introduces the latest and greatest startup scheme, launchd . It has launch-on-demand capabilities and also supports on-demand launching via Mach ports (as does the mach_init.d scheme). launchd also offers the ability to launch on demand based on file system and Unix domain socket events. The property list (.plist) files for system-installed daemons are in /System/Library/LaunchDaemons. Locally-installed daemons can be installed into /Library/LaunchDaemons. Table 4-3 lists and describes the system-installed daemons, most of which have counterparts in Linux and Unix systems.

For an example of a launch daemon property list, see "Periodic Jobs,” later in this chapter.

You can control launch daemons with the launchctl utility. For example, to enable and load a daemon that’s disabled (there will be a Disabled key in its property list file), use launchctl load -w followed by the path to the property list. For example, the following command would enable and start the telnet server:

    # launchctl load -w /System/Library/LaunchDaemons/telnet.plist

You can stop and disable this daemon with unload -w:

    # launchctl unload -w /System/Library/LaunchDaemons/telnet.plist

For more information, see the launchctl manpage.

Table 4-3. Default Mac OS X launch daemons

Property List File	Description	Enabled by default?
bootps.plist	Starts the DHCP/BOOTP daemon.	No
com.apple.atrun.plist	Launches the atrun daemon.	Yes
com.apple.KernelEventAgent.plist	Runs the kernel event agent, which responds to low-level kernel events (such as disk and network events).	Yes
com.apple.mDNSResponder.plist	Starts the Multicast DNS responder, needed by Bonjour.	Yes
com.apple.nibindd.plist	Launches the NetInfo binder daemon.	Yes
com.apple.periodic-daily.plist	Runs the daily periodic job.	Yes
com.apple.periodic-monthly.plist	Runs the monthly periodic job.	Yes
com.apple.periodic-weekly.plist	Runs the weekly periodic job.	Yes
com.apple.portmap.plist	Starts the portmapper.	Yes
com.apple.syslogd.plist	Launches the system log daemon.	Yes
com.apple.xgridagentd.plist	Runs the Xgrid agent.	No
com.apple.xgridcontrollerd.plist	Runs the Xgrid controller.	No
com.vix.cron.plist	Starts the cron daemon.	Yes
eppc.plist	Runs the Apple Events server.	No
exec.plist	Starts rexecd, the remote execution server.	No
finger.plist	Launches the finger daemon.	No
ftp.plist	Starts the FTP server.	No
login.plist	Starts the remote login (rlogin) daemon.	No
nmbd.plist	Launches Samba’s nmbd daemon.	No
ntalk.plist	Starts the ntalk daemon.	No
org.isc.named.plist	Runs named.	No
org.postfix.master.plist	Launches the postfix master process.	Yes
org.xinetd.xinetd.plist	Starts the Internet superserver (xinetd).	Yes
printer.plist	Starts the CUPS lpd server.	No
shell.plist	Starts the remote shell daemon (rshd).	No
smbd.plist	Launches Samba’s smbd daemon.	No
ssh.plist	Starts the SSH server.	No
swat.plist	Runs the Samba Web Administration Tool.	No
telnet.plist	Launches the telnet server.	No
tftp.plist	Starts the Trivial FTP server daemon.	No

SystemStarter

SystemStarter examines /System/Library/StartupItems and /Library/StartupItems for applications that should be started at boot time. /Library/StartupItems contains items for locally installed applications. /System/Library/StartupItems contains items for the system. You should not modify these or add your own items here. Table 4-4 lists Mac OS X’s available startup items.

Because much of SystemStarter’s responsibilities have been handled by launchd, the number of startup items has dramatically decreased since Mac OS X 10.3.

Table 4-4. Mac OS X default startup items

Item	Description
Apache	Starts the Apache web server. Enable this with the `WEBSERVER` entry in/etc/hostconfig or by turning on Web Sharing (System Preferences → Sharing).
AppServices	Starts the desktop database, input managers, and printing services.
AppleShare	Starts Apple file sharing. Enable this with the `AFPSERVER` entry in /etc/hostconfig or by turning on File Sharing (System Preferences → Sharing).
AuthServer	Starts the authentication server. Enable this with the `AUTHSERVER` entry in /etc/hostconfig.
CrashReporter	Enables automatic crash report generation when an application crashes. Enable this with the `CRASHREPORTER` entry in /etc/hostconfig.
Disks	Mounts local filesystems.
FibreChannel	Starts support for Fibre Channel controllers.
IFCStart	Launches ifcstart, which is used for international components of Mac OS X.
IPServices	Starts xinetd and, optionally, Internet address sharing.
Metadata	Launches the daemons required for Spotlight.
NFS	Starts the NFS client. The NFS server is started if NetInfo or /etc/exports has been configured to export one or more filesystems.
NIS	Starts the Network Information Service unless `NISDOMAIN` is set to `-NO` in /etc/hostconfig.
NetworkTime	Starts the NTP client. Enable this with the `TIMESYNC` entry in /etc/hostconfig or with System Preferences → Date & Time.
PrintingServices	Starts the Common Unix Printing System (CUPS).
RemoteDesktopAgent	Starts the remote desktop server. Enable it with the `ARDAGENT` entry in /etc/hostconfig or by enabling Apple Remote Desktop in System Preferences → Sharing.
SNMP	Starts snmpd, the SNMP daemon. Enable it with the `SNMPSERVER` entry in /etc/hostconfig.

The Login Window

Once SystemStarter is finished, getty is launched. In /etc/ttys, the console entry launches the Login Window (/System/Library/CoreServices/loginwindow.app ). At this point, the system is fully functional and ready to accept logins.

Adding Startup Items

To automatically start applications, you have two choices: start them when a user logs in, or start them when the system boots up. On most Unix systems, startup applications either reside in the /etc/rc.local script or the /etc/init.d directory. Under Mac OS 9, you could add a startup item by putting its alias in System Folder/Startup Items. Mac OS X has a different approach, described in the following sections.

Login Preferences

To start an application each time you log in, use the Accounts panel of System Preferences and select the Login Items tab. This is good for user applications, such as Stickies or an instant messenger program. For system daemons, you should set up a directory in /Library/StartupItems, as described in the next section.

Startup Items

If you compile and install a daemon, you’ll probably want it to start at boot time. For example, MySQL will build out of the box on Mac OS X (you can download it from http://www.mysql.com).

Warning

In some cases, you can start a daemon by creating a launch daemon property list in /Library/LaunchDaemons. However, there are many restrictions on launch daemons—for example, they are not allowed to change the user or group id. Also, launch daemons do not have a facility for shutting down. For complete details on these restrictions, see the launchd.plist manpage. If you are setting up a daemon that either cannot abide by the launchd restrictions, or one that needs to be shutdown gracefully, you should create a Startup Item as described in this section.

A startup item is controlled by three things: a folder (such as /Library/StartupItems/MyItem), a shell script with the same name as the directory (such as MyItem), and a property list named StartupParameters.plist . The shell script and the property list must appear at the top level of the startup item’s folder. You can also create a Resources directory to hold localized resources, but this is not mandatory.

To set up the MySQL startup item, create the directory /Library/StartupItems/MySQL as root. Then, create two files in that directory, the startup script MySQL and the property list StartupParameters.plist. The MySQL file must be an executable since it is a shell script. After you set up these two files as directed in the following sections, MySQL is launched at each boot.

The startup script

The startup script should be a shell script with StartService(), StopService( ), and RestartService( ) functions. The contents of /Library/StartupItems/MySQL/MySQL are shown in Example 4-3. The function call at the bottom of the script invokes the RunService() function from rc.common, which in turn invokes StartService( ), StopService( ), or RestartService( ), depending on whether the script was invoked with an argument of start, stop, or restart.

Tip

Although previous versions of Mac OS X did not invoke the StopService( ) code when the system was shut down, Mac OS X Tiger does. Database developers everywhere can breathe a sigh of relief.

Example 4-3. A MySQL startup script

    #!/bin/sh

    # Source common setup, including hostconfig.
    #
    . /etc/rc.common

    StartService( )
    {
        # Don't start unless MySQL is enabled in /etc/hostconfig
        if [ "${MYSQL:=-NO-}" = "-YES-" ]; then
            ConsoleMessage "Starting MySQL"
            /usr/local/mysql/bin/mysqld_safe --user=mysql &
        fi
    }

    StopService( )
    {
        ConsoleMessage "Stopping MySQL"
        # If you've set a root password within mysql, you may
        # need to add --password=password on the next line.
        /usr/local/mysql/bin/mysqladmin shutdown
    }

    RestartService( )
    {
        # Don't restart unless MySQL is enabled in /etc/hostconfig
        if [ "${MYSQL:=-NO-}" = "-YES-" ]; then
            ConsoleMessage "Restarting MySQL"
            StopService
            StartService
        else
            StopService
        fi
    }

    RunService "$1"

Because it consults the settings of the $MYSQL environment variable, the startup script won’t do anything unless you’ve enabled MySQL in the /etc/hostconfig file. To do this, add the following line to /etc/hostconfig:

    MYSQL=-YES-

Mac OS X does not recognize any special connections between hostconfig entries and startup scripts. Instead, the startup script sources the /etc/rc.common file, which in turn sources hostconfig. The directives in hostconfig are merely environment variables, and the startup script checks the value of the variables that control its behavior (in this case, $MYSQL).

The property list

The property list (StartupParameters.plist) can be in XML or NeXT format , and the list contains attributes that describe the item and determine its place in the startup sequence. The NeXT format uses NeXTSTEP-style property lists, as shown in Example 4-4.

Example 4-4. The MySQL startup parameters as a NeXT property list

    {
      Description     = "MySQL";
      Provides        = ("MySQL");
      Requires        = ("Network");
      OrderPreference = "Late";
    }

The XML format adheres to the PropertyList.dtd Document Type Definition (DTD) . You can use your favorite text editor or the Property List Editor (/Developer/Applications/Utilities) to create your own property list. Example 4-5 shows the property list in XML.

Example 4-5. The MySQL startup parameters as an XML property list

    <?xml version="1.0" encoding="UTF-8"?>
    <!DOCTYPE plist
      SYSTEM "file://localhost/System/Library/DTDs/PropertyList.dtd">
    <plist version="0.9">
    <dict>
        <key>Description</key>
        <string>MySQL</string>
        <key>Provides</key>
        <array>
            <string>MySQL</string>
        </array>
        <key>Requires</key>
        <array>
            <string>Network</string>
        </array>
        <key>OrderPreference</key>
        <string>Late</string>
    </dict>
    </plist>

The following list describes the various keys you can use in a startup parameters property list:

Description: This is a phrase that describes the item.
Provides: This is an array of services that the item provides (for example, Apache provides Web Server). These services should be globally unique. In the event that SystemStarter finds two items that provide the same service, it starts the first one it finds.
Requires: This is an array of services that the item depends on. It should correspond to another item’s Provides attribute. If a required service cannot be started, the system won’t start the item.
Uses: This is similar to Requires, but it is a weaker association. If SystemStarter can find a matching service, it will start it. If it can’t, the dependent item still starts.
OrderPreference: The Requires and Uses attributes imply a particular order, in that dependent items will be started after the services they depend on. You can specify First, Early, None (the default), Late, or Last here. SystemStarter does its best to satisfy this preference, but dependency orders prevail.

You can now manually start, restart, and stop MySQL by invoking SystemStarter from the command line:

    $ sudo SystemStarter start MySQL
    $ sudo SystemStarter restart MySQL
    $ sudo SystemStarter stop MySQL

Scheduling Tasks

Like other flavors of Unix, Mac OS X uses cron to schedule tasks for periodic execution. Each user’s cron jobs are controlled by configuration files that you can edit with crontab -e. (To list the contents of the file, use crontab -l.)

Periodic Jobs

In Mac OS X Tiger, the global crontab (/etc/crontab) has been replaced with three launch daemons. The original crontab looked like this:

    15 3 * * *       root    periodic daily
    30 4 * * 6       root    periodic weekly
    30 5 1 * *       root    periodic monthly

But now, each line is replaced by a file in /System/Library/LaunchDaemons (com.apple.periodic-daily.plist, com.apple.periodic-weekly.plist, and com.apple.periodic-monthly.plist) that uses the StartCalendar tag to specify when it is to be run. For example, here is the com.apple.periodic-daily.plist file:

    <?xml version="1.0" encoding="UTF-8"?>
    <!DOCTYPE plist PUBLIC "-//Apple Computer//DTD PLIST 1.0//EN" "http://www.apple.
    com/DTDs/PropertyList-1.0.dtd">
    <plist version="1.0">
    <dict>
            <key>Label</key>
            <string>com.apple.periodic-daily</string>
            <key>ProgramArguments</key>
            <array>
                    <string>/usr/sbin/periodic</string>
                    <string>daily</string>
            </array>
            <key>LowPriorityIO</key>
            <true/>
            <key>Nice</key>
            <integer>1</integer>
            <key>StartCalendarInterval</key>
            <dict>
                    <key>Hour</key>
                    <integer>3</integer>
                    <key>Minute</key>
                    <integer>15</integer>
            </dict>
    </dict>
    </plist>

These three launch daemons run the scripts contained in subdirectories of the /etc/periodic directory: /etc/periodic/daily, /etc/periodic/weekly, and /etc/periodic/monthly. Each of these directories contains one or more scripts:

    /etc/periodic/daily/100.clean-logs
    /etc/periodic/daily/500.daily
    /etc/periodic/monthly/500.monthly
    /etc/periodic/weekly/500.weekly

By default, the launch daemons runs them in the wee hours of the night. If your Mac is not usually turned on at those times, you could either edit the com.apple.periodic-*.plist files or remember to run them periodically using the following syntax:

    sudo periodic daily weekly monthly

As you’ll see in Chapter 5, it is vitally important that you run these jobs to ensure that your local NetInfo database is backed up.

You should not modify these files, because they may be replaced by future system updates. Instead, create a /etc/daily.local, /etc/weekly.local, or /etc/monthly.local file to hold your site-specific cron jobs. The cron jobs are simply shell scripts that contain commands to be run as root. The local cron jobs are invoked at the end of the 500.daily, 500.weekly, and 500.monthly scripts found in the /etc/periodic subdirectory.

^[*]Mach is a microkernel operating system developed at Carnegie Mellon University. The Mac OS X kernel, xnu, is a hybrid of Mach and BSD.

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Mac OS X Tiger for Unix Geeks, 3rd Edition by Ernest E. Rothman, Brian Jepson