At, atq and atrm Commands
Allow jobs to be executed in the future at a predefined date or time. Generally the "at" command is used to schedule your job/script, "atq" is used to show jobs waiting to run and "atrm" is used to remove jobs from the queue. At is used normally when you need to run a job only once at a certain time/date.
at命令是一次性定时计划任务，at的守护进程atd会以后台模式运行，检查作业队列来运行作业。atd守护进程会检查系统上的一个特殊目录来获取at命令的提交的作业，默认情况下，atd守护进程每60秒检查一次目录，有作业时，会检查作业运行时间，如果时间与当前时间匹配，则运行此作业。
注意：at命令是一次性定时计划任务，执行完一个任务后不再执行此任务了。
Atq 或at -l 显示任务。带着任务号。atrm 可以用来删除任务。用atq显示的任务号。
You can schedule an at job in two different ways:
- Schedule the job to be executed at a specific time. For example, July 3rd, 10AM
- Schedule the job to be executed in relative time from now. For example, 5 hours from now.
1. Schedule an at job using specific date and time
Syntax:
$ at time date
For example, to schedule a job at 11 am on May 20, use the following at command.
$ at 11 am may 20
2. Schedule an at job using relative time
You can schedule a job to be executed using relative time from now.
Syntax:
$ at now + COUNT UNIT
For example, following job will be execute 1 minute from now.
$ at now + 1 min
The above example will read the commands from stdin, and it will execute the job after a minute. When you give something wrong in time format, you will get the error ‘Garbled time‘.
You can schedule a background job for 1 hour from now, (or) 1 day from now using the following at command:
$ at now + 1 hour
$ at now + 1 day
3. View all the scheduled at jobs using atq
You can use atq command (or at -l), to display all the at command jobs that are scheduled or currently running.
The following atq command will list all the pending at jobs. The first number shown is the Job number, followed by the time in which the process is to be executed, and the user name.
$ atq
4 2010-04-20 11:00 a sathiya
4. Remove/Delete a scheduled at job using atrm
You can use atrm command (or at -d), to delete a particular job. For example, to delete the job number 4, use the following atrm command.
$ atrm 4
5. Execute a job only when system load average is < 1.5 using batch command
You can schedule a job using batch command, which will prompt for command input, which will be executed when the system load average is less than 1.5.
$ batch
At the successful completion of input, you will get job number. For listing and removing batch jobs you can use the at commands explained above.
6. Schedule at jobs from file using -f option
First create a text file that contains all the commands, or shell-scripts that you would like to be executed in the background using at command.
$ cat myjobs.txt
/home/sathiya/calculate-space.sh
/path/to/a/shell-script
/path/to/any/command/or/script
Using the -f option, you can make the at command to get the input from the file instead of stdin.
Following at command will execute all the jobs from the myjobs.txt 1 hour from now.
$ at -f myjobs.txt now + 1 hour
7. Allowing and Denying certain users from using at jobs
System administrator can control who can schedule an at job and who cannot using at.allow and at.deny files.
First, system checks for at.allow file. If at.allow exists, only the usernames specified in the at.allow file are allowed to use at command.
Next, (if at.allow doesn’t exist), system checks for at.deny file. If at.deny exist, the usernames specified in the at.deny file are not allowed to use the at command.
By default, most systems uses at.deny file to stop certain users from using the at command, such as www-data, guest, backup, man user.
8. Execute at command like nohup
Similar to the we discussed earlier, you can execute a command (or shell script) on the remote server using the at command and logout from the server.
$ at -f myjob now + 1 min
$ exit
Note: myjob will still be running even after you exit out of the server.
9. Additional at command time formats
You can use any one of the following at command date time formats:
$ at 10 am tomorrow
$ at 11:00 next month
$ at 22:00 today
$ at now + 1 week
$ at noon
sh-3.2# whoami
root
sh-3.2# whereis at
/usr/bin/at
sh-3.2# whereis atq
/usr/bin/atq
sh-3.2# whereis atrm
/usr/bin/atrm
sh-3.2#
--------------------------------------------
sh-3.2# cat ./tmp_at.sh
#!/bin/sh
echo " My Name is written in water.">tmp.txt
sh-3.2# at -f ./tmp_at.sh 1:51 pm
sh-3.2# cat tmp.txt
My Name is written in water.
sh-3.2# date
Tue Jan 14 13:51:30 CST 2020