At

Christian Külker

0.1.0

2023-05-01

Overview

The at command is a tool that allows you to schedule tasks to run at a later time. It reads commands from standard input or a specified file and executes the commands at a specified time. The at command is part of the at package and is included in most Debian-based distributions.

Debian Package Versions

Distribution Package Version
Stretch 3.1.20-3
Buster 3.1.23-1
Bullseye 3.1.23-1.1

To install the at package, run:

aptitude update
aptitude install at

Usage

The command is executed by the local user and the errors, if any, are displayed in a mail to the local user. Or, if the command runs successfully, the output (STDOUT) is sent to the local user via email.

Example 1: Backup a directory

You can use the at command to schedule a one-time backup of a directory at a specific time. To backup /home/$USER/Documents to /home/$USER/Backup at 22:00, you can run

echo "tar -czvf /home/$USER/Backup/Doc.tar.gz /home/$USER/Documents"|at 22:00

This will generate the following mail

Date: Mon, 01 May 2023 12:12:00 +0200
From: $USER <$USER@example.org>
To: $USER@host.example.org
Subject: Output from your job        4

tar: Removing leading `/' from member names
/home/$USER/Documents/
/home/$USER/Documents/Halmak.xml
/home/$USER/Documents/Halmak-course.xml
/home/$USER/Documents/Halmak-keyboard.xml
/home/$USER/Documents/Untitled 1.ods
/home/$USER/Documents/TODO.tdo

Example 2: Send an email reminder

The at command can be used to send an email reminder at a specified time. To send an email reminder to user@example.com at 3:30 pm, you can run:

echo "echo 'I reminded you!'|mail -s 'Reminder' user@example.com"|at 3:30 PM

This should work on all systems if the recipient is a local user. If the recipient is a remote user, aka an external mail address, the mail client or system must be configured to actually send to remote hosts using that mail client and/or user account. If it is not configured that way, you will receive an error message. In this case a local user $USER@example.com tried to send mail to $USER@example.org.

Date: Mon, 01 May 2023 08:00:00 +0200
From: Mail Delivery System <Mailer-Daemon@host.example.com>
To: $USER@host.example.com
Subject: Mail delivery failed: returning message to sender

[-- Attachment #1 --]
[-- Type: text/plain, Encoding: 7bit, Size: 0.3K --]

This message was created automatically by mail delivery software.

A message that you sent could not be delivered to one or more of its
recipients. This is a permanent error. The following address(es) failed:

  $USER@example.org
    Mailing to remote domains not supported

[-- Attachment #2 --]
[-- Type: message/delivery-status, Encoding: 7bit, Size: 0.1K --]

Reporting-MTA: dns; s1

Action: failed
Final-Recipient: rfc822;$USER@example.org
Status: 5.0.0

[-- Attachment #3 --]
[-- Type: message/rfc822, Encoding: 7bit, Size: 0.5K --]

Date: Mon, 1 May 2023 08:00:00 +0200
From: "$USER ($USER@example.org)" <$USER@host.example.com>
To: $USER@example.org
Subject: Reminder
User-Agent: NeoMutt/20170113 (1.7.2)

I reminded you!

In some cases it makes more sense to use mutt instead of mail.

Example 3: System update

You can use the at command to schedule a system update at a time when system usage is low. To perform a system update at 2 a.m., you can run

echo "aptitude update" | at 2 AM

However, such a task is probably better implemented with cron.

Example 4: Download a file at a specific time

You can use the at command to schedule a one-time task to download a file at a specific time, which can be useful if you have limited bandwidth during peak hours. To download a file from https://example.com/file.tar to /home/$USER/DL tomorrow at 2:30 a.m., you can run

echo "wget -P /home/$USER/DL https://example.com/file.tar"|at 2:30 AM tomorrow

Alternative Commands

  1. Cron: The cron command is used to schedule recurring tasks. While at is useful for one-time tasks, cron is more suitable for tasks that need to be run repeatedly, such as regular backups or updates.

  2. Anacron: Similar to cron, anacron is used to schedule recurring tasks. However, anacron is designed for systems that are not running 24/7, as it can execute missed tasks when the system is back online.

Pros and Cons

Pros

  • Simple syntax for scheduling one-time tasks.
  • Can schedule tasks based on a variety of time formats.
  • Can read commands from a file or standard input.

Cons

  • Not suitable for scheduling recurring tasks.
  • Lacks advanced features found in other scheduling tools like cron and anacron.

Compiling ‘at’ from Source

To compile at from source, follow these steps:

  1. Install the required dependencies:
aptitude install build-essential automake autoconf gnupg flex sendmail

Perhaps sendmail can be replaced with another mail transfer agent or mail user client that provides a sendmail command. Also, whether autoconf and automake are really needed can be investigated. Some developer files are created with these tools, but configuration and make work with and without them, but the last line of make is different.

  1. Download the source code and signature files:
wget http://software.calhariz.com/at/at_3.2.5.orig.tar.gz
wget http://software.calhariz.com/at/at_3.2.5.orig.tar.gz.sig
  1. Verify the source code using the signature file:
gpg --verify at_3.2.5.orig.tar.gz.sig at_3.2.5.orig.tar.gz

This will usually fail. In this case, the key is not found on the local system, and the error message looks like this:

gpg: Signature made Sun 27 Feb 2022 02:26:29 PM CET
gpg:                using RSA key 464BC7CD439FEE5E8B4098A0348A778D6885EF8F
gpg: Can't check signature: No public key
  1. Import the GPG key used to sign the package
gpg --recv-keys 464BC7CD439FEE5E8B4098A0348A778D6885EF8F

Unfortunately, I could not find the key anywhere on this planet. So I skipped this step.

  1. If verification is successful on another planet, extract the source code and change to the extracted directory:
tar -xzvf at_3.2.5.orig.tar.gz
cd at-3.2.5
  1. Compile and install:

As user:

./configure
make

As root

make install

Now you have successfully compiled and installed at from source.

Documenation

man at

History

Version Date Notes
0.1.0 2023-05-01 Initial release
  • At