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。