Commands

Commands

at

[-V] [-q queue] [-f file] [-mldbv] TIME

at

-c job [job ]

atq

[-V] [-q queue] [-v]

atrm

[-V] job [job ]

batch

[-V] [-q queue] [-f file] [-mv] [TIME]

The at command suite (at, atq, atrm, batch) is a utility for executing commands at a later time. More than one command may be scheduled for execution at a time. A set of scheduled commands is called a queue.

at	Execute commands at a specified time.
atq	List the current user's queued jobs. (If superuser, list all jobs.)
atrm	Delete jobs from the queue.
batch	Run only the commands when the system load levels permit.

Specifying a Time

There are three ways to specify a time: Absolute, Relative, or Verbal.

Absolute:	MM/DD/YY or DD.MM.YY or MMDDYY (with or without HH:MM)
Relative:	<some absolute time> + <delay> where
Delay is:	<some number> [minutes|hours|days|weeks]
Verbal:	noon, midnight, teatime, AM, PM, tomorrow, +

Example: To run the script "backup.sh" at midnight, use

at midnight -f backup.sh

Example: To find out what job number the job submitted in the previous example is, type

atq

which would produce results like this one

3 1999-07-25 00:00 a

Example: To cancel execution of the script "backup.sh" as implemented in the previous examples, type

atrm 3

-V	Display version information.
-qqueue	Specify a queue. Valid values for queue are [a-z][A-Z], and = for currently running jobs. Default values are a for at and b for batch. Queues with higher letters run with increased nice values.
-m	Send mail to the user when the job finishes running.
-ffile	Commands to be run will be taken from the specified file, rather than the standard input.
-l	List queued jobs.
-d	Remove previously queued job.
-v	For atq, this command shows completed, but not yet deleted, jobs in the queue. For all others, it shows the time the job will be executed.
-c	Display the jobs listed on the command line to standard output.

crontab

[ -u user ] file

crontab

[ -u user ] { -l | -r | -e }

Crontab is the command used to maintain your list of cron jobs. A cron job is a job scheduled to be executed automatically by the system at some specified time or interval. In order for this to work, the cron daemon must be running.

Confusingly, cron jobs are stored in files that are also called crontabs. On most systems, each user can set up his or her own crontab. (It is possible to configure cron such than no one but the superuser can use cron.) If your user name is listed in a file /etc/cron.deny, you cannot use cron. Similarly, if the file /etc/cron.allow exists, you must be listed in it or you cannot use cron.

When you use crontab for the first time, your default editor (usually vi) is invoked and you're looking at a blank file. You use crontab by adding and deleting entries from this file. Entries are in the following format:

Variable:	minutes	hours	day-of-month	month	weekday	command
Range:	0-59	0-23	1-31	1-12	0-6	[any command]

When you're done editing, quit out of the editor the normal way. Cron will look at what you've done and tell you if you made any formatting errors. Otherwise, the jobs will be executed as specified. Note that for the weekdays field, 0 = sunday.

Depending on what you're trying to do with cron, some of the fields will always be blank. Always mark the unused fields with asterisks, "*".

 

Example: To edit your crontab, use

crontab -e

To run the shell script "cleanup.sh" at midnight on Fridays, add the following line to the crontab file:

0 0 * * 5 crontab.sh

You can, technically, edit your cron files without using crontab. But it isn't a good idea.

 

-uuser	Specify a crontab other than the default.
-l	List the crontab to standard output.
-r	Remove the entire crontab.
-e	Edit the crontab. Note that when you exit the editor, any changes you make will be automatically submitted to the cron daemon.

sleep

[NUMBER[smhd]]

This command is used to pause your system or to put it to "sleep" for a specified length of time. Time is specified in one of the following units:

s	seconds
m	minutes
h	hours
d	days

Example: To run the command wall < message (which displays the contents of the file "message" to the screens of all users currently logged in) after a two-minute delay, use

sleep 120; wall < message

--help	Display help information.
--version	Display version information.

usleep

[number]

This command is used to pause your system (or to put it to "sleep") for some number of microseconds.

Example: To run the command synctest twice with a delay of fifteen microseconds between executions, use

synctest; usleep 15; synctest

--help	Print help information.
-v	Print version information.