crontab用法

crontab可以定时执行某些操作，很实用。

通常Linux系统都已经自带。

• 用法

  $ crontab -h
  crontab: invalid option -- 'h'
  crontab: usage error: unrecognized option
  usage:    crontab [-u user] file
      crontab [ -u user ] [ -i ] { -e | -l | -r }
          (default operation is replace, per 1003.2)
      -e    (edit user's crontab)
      -l    (list user's crontab)
      -r    (delete user's crontab)
      -i    (prompt before deleting user's crontab)
  

• 显示当前已经添加到crontab的命令：

  $ crontab -l
  # 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
  *   0,12   *   *   *   /bin/bash /home/adam/git2/sync.sh
  

• 将默认编辑器从nano转为vim（可以直接添加到.bashrc or .zshrc中）：

  export EDITOR="/usr/bin/vim"
  

• 修改后看到参数，其含义分别是：

  second/minute/hour/date of month/month/date of week/command

  *表示缺省，即不考虑这个时间；如分钟为*，则说明任意分钟都可以。