107.2 Lesson 2

Certificate:

LPIC-1

Version:

5.0

Topic:

107 Administrative Tasks

Objective:

107.2 Automate system administration tasks by scheduling jobs

Lesson:

2 of 2

Introduction

As you learned in the previous lesson, you can schedule regular jobs using cron or systemd timers, but sometimes you may need to run a job at a specific time in the future only once. To do this, you can use another powerful utility: the at command.

Schedule Jobs with `at`

The at command is used for one-time task scheduling and only requires that you specify when the job should be run in the future. After entering at on the command line followed by the time specification, you will enter the at prompt where you can define the commands to be executed. You can exit the prompt with the Ctrl+D key-sequence.

$ at now +5 minutes
warning: commands will be executed using /bin/sh
at> date
at> Ctrl+D
job 12 at Sat Sep 14 09:15:00 2019

The at job in the above example simply executes the date command after five minutes. Similar to cron, the standard output and error is sent to you via e-mail. Note that the atd daemon will need to be running on the system in order for you to use at job scheduling.

Note	In Linux, the `batch` command is similar to `at`, however `batch` jobs are executed only when the system load is low enough to allow it.

The most important options which apply to the at command are:

-c: Print the commands of a specific job ID to the standard output.
-d: Delete jobs based on their job ID. It is an alias for atrm.
-f: Read the job from a file instead of the standard input.
-l: List the pending jobs of the user. If the user is root, all jobs of all users are listed. It is an alias for atq.
-m: Send mail to the user at the end of the job even if there was no output.
-q: Specify a queue in the form of a single letter from a to z and from A to Z (by default a for at and b for batch). Jobs in the queues with the highest letters are executed with increased niceness. Jobs submitted to a queue with a capital letter are treated as batch jobs.
-v: Show the time at which the job will run before reading the job.

List Scheduled Jobs with `atq`

Now let us schedule two more at jobs: the first executes the foo.sh script at 09:30 am, while the second executes the bar.sh script after one hour.

$ at 09:30 AM
warning: commands will be executed using /bin/sh
at> ./foo.sh
at> Ctrl+D
job 13 at Sat Sep 14 09:30:00 2019
$ at now +2 hours
warning: commands will be executed using /bin/sh
at> ./bar.sh
at> Ctrl+D
job 14 at Sat Sep 14 11:10:00 2019

To list your pending jobs, you can use the atq command which shows the following information for each job: job ID, job execution date, job execution time, queue, and username.

$ atq
14      Sat Sep  14 11:10:00 2019 a frank
13      Sat Sep  14 09:30:00 2019 a frank
12      Sat Sep  14 09:15:00 2019 a frank

Remember that the at -l command is an alias for atq.

Note	If you run `atq` as root, it will display the queued jobs for all users.

Delete Jobs with `atrm`

If you want to delete an at job, you can use the atrm command followed by the job ID. For example, to delete the job with ID 14, you can run the following:

$ atrm 14

You can delete multiple jobs with atrm by specifying multiple IDs separated by spaces. Remember that the at -d command is an alias for atrm.

Note	If you run `atrm` as root you can delete the jobs of all users.

Configure Access to Job Scheduling

Authorization for ordinary users to schedule at jobs is determined by the /etc/at.allow and /etc/at.deny files. If /etc/at.allow exists, only non-root users listed within it can schedule at jobs. If /etc/at.allow does not exist but /etc/at.deny exists, only non-root users listed within it cannot schedule at jobs (in this case an empty /etc/at.deny file means that each user is allowed to schedule at jobs). If neither of these files exist, the user’s access to at job scheduling depends on the distribution used.

Time Specifications

You can specify when to execute a particular at job using the form HH:MM, optionally followed by AM or PM in case of 12-hour format. If the specified time has already passed, the next day is assumed. If you want to schedule a particular date on which the job will run, you must add the date information after the time using one of the following forms: month-name day-of-month, month-name day-of-month year, MMDDYY, MM/DD/YY, DD.MM.YY and YYYY-MM-DD).

The following keywords are also accepted: midnight, noon, teatime (4 pm) and now followed by a plus sign (+) and a time period (minutes, hours, days and weeks). Finally, you can tell at to run the job today or tomorrow by suffixing the time with the words today or tomorrow. For example, you can use at 07:15 AM Jan 01 to execute a job at 07:15 am on 01 January and at now +5 minutes to execute a job five minutes from now. You can read the timespec file under the /usr/share tree for more information about the exact definition of time specifications.

An Alternative to `at`

Using systemd as the system and service manager, you can also schedule one-time tasks with the systemd-run command. It is typically used to create a transient timer unit so that a command will be executed at a specific time without the need to create a service file. For example, acting as root, you can run the date command at 11:30 AM on 2019/10/06 using the following:

# systemd-run --on-calendar='2019-10-06 11:30' date

If you want to run the foo.sh script, located in your current working directory, after two minutes you can use:

# systemd-run --on-active="2m" ./foo.sh

Consult the manual pages to learn all possible uses of systemd-run with systemd-run(1).

Note	Remember that timers are logged to the systemd journal and you can review the logs of the different units using the `journalctl` command. Also remember that if you are acting as an ordinary user, you need to use the `--user` option of the `systemd-run` and `journalctl` commands.

Guided Exercises

For each of the following time specifications, indicate which is valid and which is invalid for at:

at 08:30 AM next week

at midday

at 01-01-2020 07:30 PM

at 21:50 01.01.20

at now +4 days

at 10:15 PM 31/03/2021

at tomorrow 08:30 AM
Once you have scheduled a job with at, how can you review its commands?
Which commands can you use to review your pending at jobs? Which commands would you use to delete them?
With systemd, which command is used as an alternative to at?

Explorational Exercises

Create an at job that runs the foo.sh script, located in your home directory, at 10:30 am on coming October 31st. Assume you are acting as an ordinary user.
Login to the system as another ordinary user and create another at job that runs the bar.sh script tomorrow at 10:00 am. Assume the script is located in the user’s home directory.
Login to the system as another ordinary user and create another at job that runs the foobar.sh script just after 30 minutes. Assume the script is located in the user’s home directory.
Now as root, run the atq command to review the scheduled at jobs of all users. What happens if an ordinary user executes this command?
As root, delete all these pending at jobs using a single command.
Run the ls -l /usr/bin/at command and examine its permissions.

Summary

In this lesson, you learned:

Use at to run one-time jobs at a specific time.
Manage at jobs.
Configure user access to at job scheduling.
Use systemd-run as an alternative to at.

The following commands and files were discussed in this lesson:

at: Execute commands at a specified time.
atq: List the user’s pending at jobs, unless the user is the superuser.
atrm: Delete at jobs, identified by their job number.
/etc/at.allow and /etc/at.deny: Particular files used to set at restrictions.
systemd-run: Create and start a transient timer unit as an alternative to at for one-time scheduling.

Answers to Guided Exercises

For each of the following time specifications, indicate which is valid and which is invalid for at:

at 08:30 AM next week

Valid

at midday

Invalid

at 01-01-2020 07:30 PM

Invalid

at 21:50 01.01.20

Valid

at now +4 days

Valid

at 10:15 PM 31/03/2021

Invalid

at tomorrow 08:30 AM monotonic

Invalid

Once you have scheduled a job with at, how can you review its commands?

You can use the at -c command followed by the ID of the job whose commands you want to review. Note that the output also contains most of the environment that was active at the time the job was scheduled. Remember that root can review the jobs of all users.
Which commands can you use to review your pending at jobs? Which commands would you use to delete them?

You can use the at -l command to review your pending jobs, and you can use the at -d command to delete your jobs. at -l is an alias for atq and at -d is an alias for atrm. Remember that root can list and delete the jobs of all users.
With systemd, which command is used as an alternative to at?

The systemd-run command can be used as an alternative to at to schedule one-time jobs. For example, you can use it to run commands at a specific time, defining a calendar timer or a monotonic timer relative to different starting points.

Answers to Explorational Exercises

Create an at job that runs the foo.sh script, located in your home directory, at 10:30 am on coming October 31st. Assume you are acting as an ordinary user.
```
$ at 10:30 AM October 31
warning: commands will be executed using /bin/sh
at> ./foo.sh
at> Ctrl+D
job 50 at Thu Oct 31 10:30:00 2019
```
Login to the system as another ordinary user and create another at job that runs the bar.sh script tomorrow at 10:00 am. Assume the script is located in the user’s home directory.
```
$ at 10:00 AM tomorrow
warning: commands will be executed using /bin/sh
at> ./bar.sh
at> Ctrl+D
job 51 at Sun Oct 6 10:00:00 2019
```
Login to the system as another ordinary user and create another at job that runs the foobar.sh script just after 30 minutes. Assume the script is located in the user’s home directory.
```
$ at now +30 minutes
warning: commands will be executed using /bin/sh
at> ./foobar.sh
at> Ctrl+D
job 52 at Sat Oct 5 10:19:00 2019
```
Now as root, run the atq command to review the scheduled at jobs of all users. What happens if an ordinary user executes this command?
```
# atq
52      Sat Oct  5 10:19:00 2019 a dave
50      Thu Oct 31 10:30:00 2019 a frank
51      Sun Oct  6 10:00:00 2019 a emma
```
If you run the atq command as root, all pending at jobs of all users are listed. If you run it as an ordinary user, only your own pending at jobs are listed.
As root, delete all of these pending at jobs using a single command.
```
# atrm 50 51 52
```
As root, run the ls -l /usr/bin/at command and examine its permissions.
```
# ls -l /usr/bin/at
-rwsr-sr-x 1 daemon daemon 43762 Dec  1  2015 /usr/bin/at
```
In this distribution, the at command has both the SUID (the s character instead of the executable flag for the owner) and SGID (the s character instead of the executable flag for the group) bits set, which means that it is executed with the privileges of the owner and group of the file (daemon for both). This is why ordinary users are able to schedule jobs with at.

107.2 Lesson 2

Introduction

Schedule Jobs with at

List Scheduled Jobs with atq

Delete Jobs with atrm

Configure Access to Job Scheduling

Time Specifications

An Alternative to at

Guided Exercises

Explorational Exercises

Summary

Answers to Guided Exercises

Answers to Explorational Exercises

Schedule Jobs with `at`

List Scheduled Jobs with `atq`

Delete Jobs with `atrm`

An Alternative to `at`