典型的Linux系统启动需要完成的服务

• Checking and mounting of all file systems
• Updating and enabling quota on all file systems
• Setting the host name
• Configuring the loopback network device
• Loading the SELinux policy and relabelling/runand/devas necessary on boot
• Registering additional binary formats in the kernel, such as Java, Mono and WINE binaries
• Setting the system locale
• Setting up the console font and keyboard map
• Creating, removing and cleaning up of temporary and volatile files and directories
• Applying mount options from/etc/fstabto pre-mounted API VFS
• Applying sysctl kernel settings
• Collecting and replaying readahead information
• Updatingutmpboot and shutdown records
• Loading and saving the random seed
• Statically loading specific kernel modules
• Setting up encrypted hard disks and partitions
• Spawning automatic gettys on serial kernel consoles
• Maintenance of Plymouth
• Machine ID maintenance
• Setting of the UTC distance for the system clock

在sysv启动系统或者upstart启动系统中，这些服务通过脚本实现；在systemd启动系统中，这写服务由C代码完成，由配置文件控制。