The following command is scheduled to run at 14:00, which stores the filesystem's usage in a file called file_system_usage.log in a user's home directory:

$ at 14:00
warning: commands will be executed using /bin/sh
at> df > ~/file_system_usage.log
at> <EOT>
job 33 at Mon Sep 21 14:00:00 2015

When we run the at command as shown, a warning message warning: commands will be executed using /bin/sh is printed, which specifies which shell will be used to execute commands. In the next line, we will see at prompt where we can specify the list of commands to be executed at 14:00. In our case, we entered the df > ~/file_system_usage.log command, which means run the df command and save its result in the file_system_usage.log file.

Once the list of commands to be entered is finished, press the Enter key and then, in the next line, use the Ctrl + d keys to exit from at prompt. Before getting a normal shell prompt, we will see the message saying created job number and time stamp at which the job will be executed. In our case, the job number is 33 and the time stamp is Mon Sep 21 14:00:00 2015.

We can check the content of the file_system_usage.log file once the time stamp we specified is over.

We can print on stdout what is going to be executed when a particular scheduled job runs:

$ at -c 33  # Lists content of job 33

We can see that the df > ~/file_system_usage.log command will be executed. The rest of the lines specify in what environment a task will be executed.

Now, consider a job scheduled by the root user:

# at -v 4am
Mon Sep 21 04:00:00 2015

warning: commands will be executed using /bin/sh
at> reboot
at> <EOT>
job 34 at Mon Sep 21 04:00:00 2015

The job with the number 34 is scheduled by the user root. This job system will reboot at 4am.

We can schedule a script file for execution at a specific time using the -f option with the at command.

For example, we want to run the loggedin_user_detail.sh script next week at 4 pm. This script lists logged in users and what processes they are running when the script gets executed at a scheduled time. The content of the script is as follows:

$ cat  loggedin_user_detail.sh
#!/bin/bash
# Filename: loggedin_user_detail.sh
# Description: Collecting information of loggedin users

users_log_file=~/users_log_file.log
echo "List of logged in users list at time 'date'" > $users_log_file
users=('who | cut -d' ' -f1 | sort | uniq')
echo ${users[*]} >> $users_log_file

for i in ${users[*]}
do
  echo "Processes owned by user $i" >> $users_log_file
  ps u -u $i >> $users_log_file
  echo
done
$ chmod +x  loggedin_user_detail.sh  # Provide execute permission

Now, to run the preceding script at 4 pm next week, we will run the following command:

$at -f loggedin_user_detail.sh 4pm + 1 week
warning: commands will be executed using /bin/sh
job 42 at Sun Sep 27 16:00:00 2015

We can see that the job has been scheduled to run one week later.