Linux timed tasks at and crontab

1. at Disposable Tasks

1.1 Command at Installation

Reads a command from a file or standard input and executes at a later time, only once. atthe normal execution of the process requires a daemonatd.

#安装at Yum install-y at# start daemon Service ATD start #查看是否开机启动 chkconfig--list|grep ATD #设置开机启动 Chkconfig--level 235 ATD on

1.2 Use

If you do not use a pipe | or specify options -f , at the execution will be interactive and you will need to enter the command at the prompt of at:

[Email protected] ~]# at now +2 minutes #执行at并指定执行时刻为现在时间的后两分钟at > Echo Hello World >/root/a.txt #手动输入命令并回车at > <EOT> #ctrl +d End Input Job 2 at 2017-07-24 16:08 #显示任务号及执行时间

Options -l or Command atq Query task

[[email protected] ~]# atq2 2017-07-24 16:21 a root[[email protected] ~]# at-l2 2017-07-24 16:21 a root

After the arrival time, the task is executed, a new file is generated and the output of ECHO is saved.

[email protected] ~]# cat a.txt Hello World




atSpecify the time method:
1) hh:mm Hours: Minutes (the day, if the time has past, then the next day of execution)
2) midnight (Late Night), ( noon noon), teatime (Afternoon tea time, 4 o'clock in the afternoon), today , tomorrow etc.
3) 12 hour clock, time plus am (morning) or pm (PM)
4) Specify the specific execution date mm/dd/yy (month/day/year) or dd.mm.yy (day, month, year).
5) Relative timing method now + n units , now is the moment, n is the number, units is the unit (minutes, hours, days, weeks)

Create a directory like tomorrow 2:20 P.M.

[Email protected]]# at 02:20pm tomorrowat> mkdir/root/temp/xat> <eot>job one at Fri Dec 23 14:20:00 2016

An option -d or command atrm represents a delete task

[Email protected]]# at-d #删除11号任务 (previous example) [[Email protected]]# atq[[email protected]]#

You can use pipelines | or options -f to at get tasks from standard input or files

[[Email protected]~]# cat test.txt echo Hello World >/root/temp/file[[email protected]~] at-f test.txt 5pm +2 daysjob In Sat Dec 17:00:00 2016[[email protected]~]# cat test.txt|at 16:20 12/23/16job at Fri Dec 23 16:20:00 2016

atdWith two files /etc/at.allow and /etc/at.deny to determine which users in the system can use the at Set Timer task, it first checks /etc/at.allow that if the file exists, only the users listed in the file (one per row) can use at, and if not, check the file /etc/at.deny . All users who are not in this file can use at. /etc/at.denyan empty file means that at is available to all users in the system, and if the /etc/at.deny file does not exist, only superuser (root) can use at.

2. crontab

Each user in the system can have their own cron table , atd similar, crond there are two files /etc/cron.allow and /etc/cron.deny used to restrict users to use cron, the rules are atd the same as the two files.

/etc/cron.deny represents a user who cannot use the crontab command

/etc/cron.allow represents a user who can use crontab.

If two files exist at the same time, then/etc/cron.allow is preferred.

If none of the two files exist, only the Superuser can schedule the job.

Each user will generate a crontab file of their own. These files are in the/var/spool/cron directory

Crontab-u Specify a user

Crontab-l List A user's task schedule

Crontab-r Delete a user's task

CRONTAB-E editing a user's tasks

/etc/cron.{ Hourly,daily,weekly,monthly} , the files in the directory define the hourly, daily, weekly, monthly scripts that need to be run, and the exact time to run these tasks is specified in the file /etc/crontab . such as:

[[email protected] spool]# cat /etc/crontabshell=/bin/bashpath=/sbin:/bin:/usr/sbin:/usr/ binmailto=roothome=/# for details see man 4 crontabs# example of  job definition:# .---------------- minute  (0 - 59) # |  .--------- ---- hour  (0 - 23) # |  |  .---------- day of month   (1 - 31) # |  |  |  .------- month  (1 -  ( OR JAN,FEB,MAR,APR ...# |  |  |  | &NBSP):----  day of week  (0 - 6)   (sunday=0 or 7)  or sun,mon,tue,wed , thu,fri,sat# |  |  |  |  |# *  *  *   *  * user-name command to be executed# run-parts01 *  * * * root run-parts /etc/cron.hourly02 4 * * * root run-parts / Etc/cron.daily22 4 * * 0 root run-parts /etc/cron.weekly42 4 1  * * root run-parts /etc/cron.monthly The first part represents the minute (0-59),*  means the second part of each minute represents the hour (0-23), *   indicates that the third part of the hour represents the day (1-31),  *  means that the Part IV represents the month (1-12),  *  that the fifth part of the month represents the week (0-6,0 represents Sunday), *   represents the task to be performed in Part VI of the week

The purpose of

Anacron is not to completely replace Cron as a supplement to cron . The task definition for Anacron is in the file /etc/anacrontab :

[[email protected] spool]# cat /etc/anacrontab # /etc/anacrontab:  Configuration file for anacron# see anacron (8)  and anacrontab (5)  for  details. shell=/bin/shpath=/sbin:/bin:/usr/sbin:/usr/binmailto=root# the maximal random delay  Added to the base delay of the jobsrandom_delay=45# the jobs  will be started during the following hours onlystart_hours_range=3-22 #period  in days   delay in minutes   job-identifier    command1       5        cron.daily              nice  run-parts /etc/cron.daily7       25       cron.weekly             nice run-parts /etc/cron.weekly@ monthly 45     cron.monthly             nice run-parts /etc/cron.monthly

This article from "High-rise, look at the end of the Road" blog, please be sure to keep this source http://nxyboy.blog.51cto.com/10511646/1950598

Linux timed tasks at and crontab