正常情况下，还是建议使用shutdown吧，它不仅可以定时，还会在关机做更多的准备工作。

2、（个人推测）虽然执行的是同一个程序，但是程序在实际运行过程中可以根据不同的程序名称（当作参数）执行不同的操作。

1. http://upstart.ubuntu.com/
Upstart is an event-based replacement for the /sbin/init daemon
Ubuntu 14.04 預設使用 Upstart 開機初始化
Upstart 被設計來取代 /sbin/init
意思是 在 Ubuntu 14.04 應該不需考慮 init
1-1. http://manpages.ubuntu.com/manpages/tru ... down.8.htm
shutdown - bring the system down
This is performed by emitting the runlevel(7) event, which includes the new runlevel in the RUNLEVEL environment variable as well as the previous runlevel (obtained from the environment or from /var/run/utmp) in the PREVLEVEL variable

2. http://manpages.ubuntu.com/manpages/tru ... oot.8.html
reboot, halt, poweroff 都是 Upstart 套件提供的指令
2-1. When called with --force or when in runlevel 0 or 6, this tool invokes the reboot(2) system call itself (with REBOOTCOMMAND argument passed) and directly reboots the system. Otherwise this simply invokes the shutdown(8) tool with the appropriate arguments without passing REBOOTCOMMAND argument.
意思是 如果沒有加 --force 參數 或 在 runlevel 0 or 6 執行
它們預設都是用來啟動 shutdown 指令

3. 建議 直接啟用 shutdown 就好
http://manpages.ubuntu.com/manpages/tru ... own.8.html
shutdown - bring the system down

Ubuntu 15.04 开始使用 systemd 接管，使用 systemctl 来控制关机也是不错选择！

例如：systemctl poweroff

Using Systemd to Power Off / Reboot / Suspend GNOME

Not everyone bothers with manuals, so I am re-printing the documentation of some nice to use commands.

Command Description
halt Shut down and halt the system.
This is mostly equivalent to start halt.target –irreversible, but also prints a wall message to all users. If combined with –force, shutdown of all running services is skipped, however all processes are killed and all file systems are unmounted or mounted read-only, immediately followed by the system halt.

If –force is specified twice, the operation is immediately executed without terminating any processes or unmounting any file systems. This may result in data loss.

poweroff Shut down and power-off the system.
This is mostly equivalent to start poweroff.target –irreversible, but also prints a wall message to all users. If combined with –force, shutdown of all running services is skipped, however all processes are killed and all file systems are unmounted or mounted read-only, immediately followed by the powering off.

If –force is specified twice, the operation is immediately executed without terminating any processes or unmounting any file systems. This may result in data loss.

reboot Shut down and reboot the system.
This is mostly equivalent to start reboot.target –irreversible, but also prints a wall message to all users. If combined with –force, shutdown of all running services is skipped, however all processes are killed and all file systems are unmounted or mounted read-only, immediately followed by the reboot.

If –force is specified twice, the operation is immediately executed without terminating any processes or unmounting any file systems. This may result in data loss.

suspend Suspend the system.
This will trigger activation of the special suspend.target target.

hibernate Hibernate the system.
This will trigger activation of the special hibernate.target target

hybrid-sleep Hibernate and suspend the system.
This will trigger activation of the special hybrid-sleep.target target.