計劃任務和周期任務mail,at,batch,atq, atrm, cron, crontab

Linux上的任務計劃：           

            任務計劃：

                        在未來時間點一次性執行某任務：at,batch

                        周期性執行某任務:crontab 

            電子郵件服務：

                        smtp: simple mailtransmission protocol

                                    /var/spool/mail/USERNAME

                        pop3: post officeprotocol

                        imap4: internet mailaccess protocol

                        MUA：mail指令

                        mail指令：

                                    mail: 進入互動式收發郵件接口；

                                    mail -s'SUBJECT' username@hostname

                                    mail -s'SUBJECT' username@hostname < /PATH/FROM/SOMEFILE 

                                    COMMAND |mail -s 'SUBJECT' username@hostname 

            at  指令： 

                        承載未來時間運作的某作業： 

                        支援使用作業隊列：

                                    預設為a隊列； 

                        Ctrl+d 

                        at [option]... TIME 

                        TIME:

                                    (1) 絕對時間

                                    HH:MM,

                                    MMDD[CC]YY,MM/DD/[CC]YY, DD.MM.[CC]YY or [CC]YY-MM-DD

                                                tomorrow

                                    (2) 相對時間

                                                now+#UNIT

                                                            minute,hour, day, week

                                    (3) 模糊時間

                                                midnight

                                                noon

                                                teatime 

                        常用選項：

                                    -q QUEUE：at作業隊列；

                                    -f/PATH/FROM/SOMEFILE：從指定的檔案中讀取要運作的作業；

                                    -l: 檢視作業隊列中的等運作作業清單；相當于使用atq指令；

                                    -cAT_JOB_NUM: 檢視待運作作業的内容；

                                    -d: 删除指定作業；相當于atrm 

            batch:

                        由系統自行選擇在資源較為空閑時運作指定的任務；

            crontab：周期性任務計劃

                        守護程序：crond 

                        周期性任務有兩類：

                                    (1) 系統cron任務；沒有預設運作使用者身份，是以需要額外指定運作者；

                                                /etc/crontab 

                                                vim指令 

                                                            #Example of job definition:

                                                            #.---------------- minute (0 - 59)

                                                            #|  .------------- hour (0 - 23)

                                                            # |  | .---------- day of month (1 - 31)

                                                            #|  | |  .------- month (1 - 12) ORjan,feb,mar,apr ...

                                                            #|  | |  |  .---- day of week (0 - 6) (Sunday=0 or 7) ORsun,mon,tue,wed,thu,fri,sat

                                                            #|  | |  |  |

                                                            #*  * *  *  * user-name command to be executed    

                                                            7個字段：

                                                                        前5個字段：時間點

                                                                        user-name:以哪個使用者的身份運作任務

                                                                        commandto be executed: 要運作任務                                 

                                    (2) 使用者cron任務：由某個使用者所送出，預設就以送出者的身份運作，是以無需額外指定運作者；

                                                /var/spool/cron/USERNAME 

                                                vim指令；不建議

                                                crontab指令：建議

                                                            #|  | .---------- day of month (1 - 31)

                                                            #*  * *  *  * command to be executed        

                                                            5個時間點：

                                                                        分鐘：有效取值範圍0-59；

                                                                        小時：0-23

                                                                        日：1-31

                                                                        月：1-12

                                                                        周幾：0-7

                                                                        注意：幾月幾号與周幾，不建議同時使用；

                                                                        例如：

                                                                                    6* * * * 

                                                                        時間表示法：

                                                                                     *:對應時間點有效取值範圍内的每個時間點；

                                                                                    -:一個特定連續時間範圍,3-7

                                                                                    ,：一個離散的時間點,3,5,7

                                                                                    /#：有效時間範圍内的每多少時間；用于指定頻率；

                                                                                                1-30/4,*/4 

                                                                        5*/3 * * * /bin/echo "howdy" 

                                                                        57 * * 1-5 /bin/echo "howdy" 

                                                            crontab指令：

                                                                        crontab[-u user] [-l | -r | -e]

                                                                                    -uuser: 不是管理自己的cron任務，而是指定的目标使用者的cron任務；僅root擁有管理其它使用者的cron任務的權限；預設管理自己的；

                                                                                    -l:list，列出任務；

                                                                                    -r:remove, 移除所有任務；

                                                                                    -e:edit，編輯，打開一目前shell會話的預設編輯器來編輯cron任務表； 

                                                思考：

                                                            1、如果想實作每8分鐘運作一次某任務？

                                                            2、如果想實作每10秒鐘運作一次某任務？

                                    補充性的工具：anacron:

                       注意：

                       (1)如果不想接收任務執行結果的通知郵件：

                                                            COMMAND> /dev/null

                                                            COMMAND&> /dev/null

                      (2)對于crontab檔案來講，%有特殊功用；如果指令中會出現%,要記得轉義，或者用單引号對其引用；

                      (3)crontab的PATH變量與使用者的變量不完全相同，是以，建議在cron中的任務要使用絕對路徑

                                                            /root/bin/a.sh