Linux at, batch, atq, and atrm commands
On Unix-like operating systems, the at, batch, atq, and atrm commands can schedule a command or commands to be executed at a specified time in the future.
The at command schedules a command to run once at a particular time you normally have permission to run. The at command can be anything from a simple reminder message, to a complex script. You start by running the at command at the command line, passing it the scheduled time as the option. It then places you at a special prompt, where you can type in the command (or series of commands) to run at the scheduled time. When you're done, press Control-D on a new line, and your command will be placed in the queue.
A typical at command sequence looks like this (commands you type are shown here in the blue box, or in bold face below):
at 9:30 PM Tue
warning: commands will be executed using /bin/sh at> echo "It's 9:30 PM on Sunday." at> ^D job 1 at Sun Nov 16 09:30:00 2014
When we ran the command, the first thing at did was give us a "warning" telling us what command shell our commands run with: /bin/sh, the Bourne Shell. This shell is the traditional standard Unix shell.
It then places us at the at> prompt. Here we type a simple echo command, which echoes a string of text. We press Enter, and we're placed at a new at> prompt. We then press Control-D, telling at we're all done with our commands. It then tells us that our job is job number 1 and that it runs next Tuesday.
The output of your specified command is mailed to you. You can read this mail with the mail program or a program like pine (or the modern version of pine, called alpine). You can download these programs if you don't have them, or install them with your package manager. For example, on Ubuntu, which uses the APT package management system, you can install them using the apt-get command, specifically: sudo apt-get install mail or sudo apt-get install alpine.
at uses a very casual representation of time and date. It even knows some "commonly used" times you might not expect — it knows that "teatime" is traditionally at 4 PM, for instance.
Here are examples of times you can pass to at to schedule a command. For instance, let's assume the current time is 10:00 AM, Tuesday, October 18, 2014. The following expressions would translate to the following times:
|the expression:||would translate to:|
|noon||12:00 PM October 18 2014|
|midnight||12:00 AM October 19 2014|
|teatime||4:00 PM October 18 2014|
|tomorrow||10:00 AM October 19 2014|
|noon tomorrow||12:00 PM October 19 2014|
|next week||10:00 AM October 25 2014|
|next monday||10:00 AM October 24 2014|
|fri||10:00 AM October 21 2014|
|NOV||10:00 AM November 18 2014|
|9:00 AM||9:00 AM October 19 2014|
|2:30 PM||2:30 PM October 18 2014|
|1430||2:30 PM October 18 2014|
|2:30 PM tomorrow||2:30 PM October 19 2014|
|2:30 PM next month||2:30 PM November 18 2014|
|2:30 PM Fri||2:30 PM October 21 2014|
|2:30 PM 10/21||2:30 PM October 21 2014|
|2:30 PM Oct 21||2:30 PM October 21 2014|
|2:30 PM 10/21/2014||2:30 PM October 21 2014|
|2:30 PM 21.10.14||2:30 PM October 21 2014|
|now + 30 minutes||10:30 AM October 18 2014|
|now + 1 hour||11:00 AM October 18 2014|
|now + 2 days||10:00 AM October 20 2014|
|4 PM + 2 days||4:00 PM October 20 2014|
|now + 3 weeks||10:00 AM November 8 2014|
|now + 4 months||10:00 AM February 18 2015|
|now + 5 years||10:00 AM October 18 2019|
So, if you run the command:
at now + 10 years
Then enter a command at the at> prompt, press Enter, and type Control-D, you are mailed the results of your command ten years from now.
If you don't specify a time at the command line, at returns the following error message:
No job is added to the queue. So, always specify your time at the command line.
Using atq to view your at queue
You can use the program atq to view your currently-queued at jobs. Type atq to display the queue.
1 Fri Oct 22 09:48:00 2014 a hope
This information is, from left to right: job number, date, hour, year, queue, and username.
The atq program only list jobs that belong to you — unless you are the superuser, where it lists the jobs of all users. So to list all at jobs currently queued on the system, type this command (if you have superuser privileges):
Type your password, when prompted.
at [-V] [-q queue] [-f file] [-mMlv] timespec...
at [-V] [-q queue] [-f file] [-mMkv] [-t time]
at -c job [job...]
atq [-V] [-q queue]
at [-rd] job [job...]
atrm [-V] job [job...]
at executes commands at a specified time.
atq lists the user's pending jobs, unless the user is the superuser; in that case, everybody's jobs are listed. The format of the output lines (one for each job) is: job number, date, hour, year, queue, and username.
atrm deletes jobs, identified by their job number.
batch executes commands when system load levels permit; in other words, when the load average drops below 1.5, or the value specified in the invocation of atd.
At allows fairly complex time specifications, extending the POSIX.2 standard. It accepts times of the form HH:MM to run a job at a specific time of day. (If that time is already past, the next day is assumed.) You may also specify midnight, noon, or teatime (4 P.M.) and you can have a time-of-day suffixed with AM or PM for running in the morning or the evening. You can also say what day the job runs by giving a date in the form month-name day with an optional year, or giving a date of the form MMDD[CC]YY, MM/DD/[CC]YY, DD.MM.[CC]YY or [CC]YY-MM-DD. The specification of a date must follow the specification of the time of day. You can also give times like now + count time-units, where the time-units can be minutes, hours, days, or weeks and you can tell at to run the job today by suffixing the time with today and to run the job tomorrow by suffixing the time with tomorrow.
For example, to run a job at 4 P.M. three days from now, you would do at 4pm + 3 days, to run a job at 10:00am on July 31, you would do at 10am Jul 31 and to run a job at 1am tomorrow, you would do at 1am tomorrow.
The definition of the time specification is found in /usr/share/doc/at/timespec.
For both at and batch, commands are read from standard input or the file specified with the -f option and executed. The working directory, the environment (except for the variables BASH_VERSINFO, DISPLAY, EUID, GROUPS, SHELLOPTS, TERM, UID, and _) and the umask are retained from the time of invocation.
As at is currently implemented as a setuid program, other environment variables (e.g., LD_LIBRARY_PATH or LD_PRELOAD) are also not exported. This may change in the future. As a workaround, set these variables explicitly in your job.
An at or batch command run from a su shell will retain the current userid. The user is mailed standard error and standard output from the commands, if any. Mail will be sent using the command /usr/sbin/sendmail. If at is executed from a su shell, the owner of the login shell will receive the mail.
The superuser may always use these commands. For other users, permission to use at is determined by the files /etc/at.allow and /etc/at.deny. See at.allow for details.
|-V||Prints the version number to standard error and exits successfully.|
|-q queue||Uses the specified queue. A queue designation consists of a single letter; valid queue designations range from a to z and A to Z. The a queue is the default for at and the b queue for batch. Queues with higher letters run with increased niceness. The special queue "=" is reserved for jobs that are currently running.
If a job is submitted to a queue designated with an uppercase letter, the job is treated as if it were submitted to batch at the time of the job. Once the time is reached, the batch processing rules with respect to load average apply. If atq is given a specific queue, it only shows jobs pending in that queue.
|-m||Send mail to the user when the job has completed even if there was no output.|
|-M||Never send mail to the user. In other words, execute the command, but do not notify the user of its output.|
|-f file||Reads the job from file rather than standard input.|
|-t time||Run the job at time, given in the format [[CC]YY]MMDDhhmm[.ss].|
|-l||Running at -l is the same as running atq; it displays all queued at jobs.|
|-r||Is the same as running atrm. It removes a job from the at queue.|
|-d||Is also an alias for atrm.|
|-b||Is an alias for batch.|
|-v||Shows the time the job will be executed before reading the job. Times displayed will be in the format "Thu Feb 20 14:50:00 1997".|
|-c||The cats the jobs listed on the command line to standard output.|
at -m 01:35 < my-at-jobs.txt
Run the commands listed in the 'my-at-jobs.txt' file at 1:35 AM. All output from the job is mailed to the user running the task. When this command is successfully entered, you receive a prompt similar to the example below:
commands will be executed using /bin/sh job 1 at Wed Dec 24 00:22:00 2014
This command lists each of the scheduled jobs in a format like the following:
1 Wed Dec 24 00:22:00 2003
...this is the same as running the command atq.
at -r 1
Deletes job 1. This command is the same as running the command atrm 1.
Deletes job 23. This command is the same as running the command at -r 23.
crontab — View and edit a list of jobs for the system to run at regular intervals.