cron is the daemon responsible for executing scheduled and recurring commands (every hour, every day, every week, etc.). atd deals with commands to be executed a single time, but at a specific moment in the future.
locate 程式使用；
crontab -e (其內容儲存在 /var/spool/cron/crontabs/user 檔案內) 命令編輯它。
/etc/crontab 檔案，或寫入額外的 crontab 檔案於 /etc/cron.d 資料夾內。最後兩個解決方案有其優勢，在執行命令時辦識使用者。
crontab entry describes a scheduled command with the six (or seven) following fields:
Sun、Mon等)；
/etc/crontab 檔案與其位在 /etc/cron.d/ 內，但不在使用者自己的 crontab 檔案內)；
a-b describes the interval of all the values between a and b. The syntax a-b/c describes the interval with an increment of c (example: 0-10/2 means 0,2,4,6,8,10). An asterisk * is a wildcard, representing all possible values.
範例 9.2. Sample user crontab file
#Format #min hour day mon dow command # Download data every night at 7:25 pm 25 19 * * * $HOME/bin/get.pl # 8:00 am, on weekdays (Monday through Friday) 00 08 * * 1-5 $HOME/bin/dosomething # every two hours * */2 * * * $HOME/bin/dosomethingelse # Restart the IRC proxy after each reboot @reboot /usr/bin/dircproxy
at executes a command at a specified moment in the future. It takes the desired time and date as command-line parameters, and the command to be executed in its standard input. The command will be executed as if it had been entered in the current shell. at even takes care to retain the current environment, in order to reproduce the same conditions when it executes the command. The time is indicated by following the usual conventions: 16:12 or 4:12pm represents 4:12 pm. The date can be specified in several European and Western formats, including DD.MM.YY (27.07.22 thus representing 27 July 2022), YYYY-MM-DD (this same date being expressed as 2022-07-27), MM/DD/[CC]YY (i.e., 12/25/22 or 12/25/2022 will be December 25, 2022), or simple MMDD[CC]YY (so that 122522 or 12252022 will, likewise, represent December 25, 2022). Without it, the command will be executed as soon as the clock reaches the time indicated (the same day, or tomorrow if that time has already passed on the same day). You can also simply write “today” or “tomorrow”, which is self-explanatory.
$at 09:00 27.07.22 <<END>echo "Don't forget to wish a Happy Birthday to Raphaël!" \>| mail lolando@debian.org>ENDwarning: commands will be executed using /bin/sh job 1 at Wed Jul 27 09:00:00 2022
at now + 數字 間隔。間隔 可以是 分、小時、日、或 週。數字 指明執行命令前的時間。
cron 排定的工作，祗需執行 crontab -e 並刪除在 crontab 檔案中對應的列。對於使用 at 的工作，同樣簡單：執行 atrm 工作編號 就可以了。排定時由 at 命令指定工作編號，可以用 atq 命令列出當前工作的清單，籨而找到該工作編號。