at (排一个或多个命令在将来的指定时间执行)

The at command schedules a command to be run once at a particular time that 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 be 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 will be run with: in this case, /bin/sh, the Bourne Shell. This shell is the traditional standard Unix shell.

It then places us at the at> prompt. Here we type in 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 will run next Tuesday.

Specifying Time

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 some 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:

...so if you run the command:

at now + 10 years

...and then enter a command at the at> prompt, press enter, and type Control-D, you will be mailed the results of your command ten years from now.

Garbled time

...and no job will be 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.

atq

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.

atq will only list jobs that belong to you — unless you are the super user, in which case it will list the jobs of all users. So to list all at jobs currently queued on the system, type this command (if you have superuser privileges):

sudo atq

...and 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...]

batch

at -b

Technical Description

at and batch read commands from standard input or a specified file that are to be executed at a later time, using sh.

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 (4pm) 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 will be run, 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 4pm 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 can be 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 will be mailed standard error and standard output from his 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.

Options

Files

/var/spool/cron/atjobs
/var/spool/cron/atspool
/proc/loadavg
/var/run/utmp
/etc/at.allow
/etc/at.deny

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 will be mailed to the user running the task. When this command has been successfully entered you should 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

at -l

This command will list 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.

atrm 23

Deletes job 23. This command is the same as running the command at -r 23.