crontab
crontab可以定时执行任务，比如可以配置为每天几点执行什么任务，其实还可以配置为开机后执行一个任务。
命令行输入crontab -e可进入编辑，每一行是一个定时任务，每个定时任务表示如下：
# ┌───────────── minute (0 - 59)
# │ ┌───────────── hour (0 - 23)
# │ │ ┌───────────── day of the month (1 - 31)
# │ │ │ ┌───────────── month (1 - 12)
# │ │ │ │ ┌───────────── day of the week (0 - 6) (Sunday to Saturday;
# │ │ │ │ │ 7 is also Sunday on some systems)
# │ │ │ │ │
# │ │ │ │ │
# * * * * * command to execute
很好理解，就是 定时时间 + 任务命令，另外有几个较为特殊的时间表示如下：
| Entry | Description | Equivalent to |
|---|---|---|
| @yearly (or @annually) | Run once a year at midnight of 1 January | 0 0 1 1 * |
| @monthly | Run once a month at midnight of the first day of the month | 0 0 1 * * |
| @weekly | Run once a week at midnight on Sunday morning | 0 0 * * 0 |
| @daily | Run once a day at midnight | 0 0 * * * |
| @hourly | Run once an hour at the beginning of the hour | 0 * * * * |
| @reboot | Run at startup | N/A |
这里用到@reboot，用法参考下面的示例。
示例
命令行输入crontab -e进入编辑：
sl@Li:~$ crontab -e
# Edit this file to introduce tasks to be run by cron.
#
# Each task to run has to be defined through a single line
# indicating with different fields when the task will be run
# and what command to run for the task
#
# To define the time you can provide concrete values for
# minute (m), hour (h), day of month (dom), month (mon),
# and day of week (dow) or use '*' in these fields (for 'any').#
# Notice that tasks will be started based on the cron's system
# daemon's notion of time and timezones.
#
# Output of the crontab jobs (including errors) is sent through
# email to the user the crontab file belongs to (unless redirected).
#
# For example, you can run a backup of all your user accounts
# at 5 a.m every week with:
# 0 5 * * 1 tar -zcf /var/backups/home.tgz /home/
#
# For more information see the manual pages of crontab(5) and cron(8)
#
# m h dom mon dow command
@reboot sh /home/sl/System/mount_share.sh
其中脚本，换成其他任务即可：
#!/bin/sh
echo asdf | sudo -S mount -t vboxsf Share /home/sl/share
编辑好后按CTRL+x，按提示保存退出即可。这样当机器重启时，就会执行挂载虚拟机共享文件夹任务。
