Scheduled Activity: cron and at
Use at when you want to execute a command or multiple commands once at some future time.
In Linux, the at command requires that the atrun command be started in root's crontab file. Many Linux distributions ship with at enabled, but some do not. To enable the at utility on your system, become superuser and edit root's crontab file:
$ su root Password: # crontab -e
and add the following line:
* * * * * directory/atrun
where directory is the location where the atrun executable is stored. On my system that's /usr/lib, so the entry is:
* * * * * /usr/lib/atrun
This causes atrun to be executed every minute. After a minute or so of adding the atrun line and saving the crontab file, any existing at commands are evaluated and executed if the time is right. (Before this, you may have been able to enter at commands, but they were never executed.)
To demonstrate the at command, let's have it print “hello” on your current terminal window after a few minutes. First, get the time and your current terminal device:
$ date Tue Oct 3 15:33:37 PDT 1995 $ tty /dev/ttyp2
Now run the at command. Specify a time in the command line, press Return, and then enter the command, followed by another Return and a Ctrl-D:
$ at 15:35 echo "hello" > /dev/ttyp2 ^D Job c00ceb20b.00 will be executed using /bin/sh
The at command takes input up to the end-of-file character (generated by pressing ctrl-D while at the beginning of a line.) It reports the job number and informs you that it will use /bin/sh to execute the command. In two minutes, hello should appear on the display of /dev/ttyp2. Note that you can enter a series of commands, one per line—at will read each line up to the end-of-file and execute the file as a /bin/sh shell script at the specified time.
Suppose you want to set an alarm. One way to tell at to do something is to use the relative form of timing, specifying a time relative to now. If you want your computer to beep at you in 25 minutes, enter:
$ at now + 25 minutes echo ^G > /dev/ttyp4 ^D Job c00ceb7fb.00 will be executed using /bin/sh
and you are beeped in 25 minutes. There is a great deal of flexibility allowed in entering time specifications. For example, at recognizes military time, “am” and “pm”, month abbreviations, time notation that includes the year, and so on. My at man page even claims at accepts teatime, noon, and other constructs. Refer to the at man page for more examples of valid time specifications.
You must tell at your tty location, or it won't send output to your terminal window. If you prefer, you can receive mail:
$ at 4:55pm Friday echo '5 p.m. meeting with Carol' | mail raithel ^D Job c00ceb7fb.01 will be executed using /bin/sh
To get a list of your pending at jobs, enter:
If you are superuser, atq shows you the pending at jobs of all users. To delete a job, enter:
$ atrm job_number
where job_number is the job number returned by atq. The superuser can also remove other user's jobs.
The following is a simple script that makes it easier for me to use at to send myself reminders. The script sends mail to the user containing the message line(s) entered at the prompt at the time specified. It also displays some syntax examples of how to specify time, which I find a useful memory refresher.
Notice the script, as written, requires you to have a Msgs directory in your home directory. I created $HOME/Msgs, rather than using something like /usr/tmp, so the messages are more private until they are deleted by the script.
#!/bin/sh echo "Enter your reminder message. When finished, enter a period (.) at the beginning of a line and press Enter. (Or press Ctrl-C or DEL to exit.)" while : do read MESSAGE if [ "$MESSAGE" = "." ] then break else echo $MESSAGE > $HOME/Msgs/message.$$ fi done cat << !! Enter time and day you want to receive the message, for example: 0815am Jan 24 8:15am Jan 24 now + 1 day 5 pm Friday Then press Enter. !! read TIME echo \ "at $TIME mail $LOGNAME $HOME/Msgs/message.$$" at $TIME << !! mail $LOGNAME < $HOME/Msgs/message.$$ rm -f $HOME/Msgs/message.$$ !! exit 0
Practical Task Scheduling Deployment
One of the best things about the UNIX environment (aside from being stable and efficient) is the vast array of software tools available to help you do your job. Traditionally, a UNIX tool does only one thing, but does that one thing very well. For example, grep is very easy to use and can search vast amounts of data quickly. The find tool can find a particular file or files based on all kinds of criteria. It's pretty easy to string these tools together to build even more powerful tools, such as a tool that finds all of the .log files in the /home directory and searches each one for a particular entry. This erector-set mentality allows UNIX system administrators to seem to always have the right tool for the job.
Cron traditionally has been considered another such a tool for job scheduling, but is it enough? This webinar considers that very question. The first part builds on a previous Geek Guide, Beyond Cron, and briefly describes how to know when it might be time to consider upgrading your job scheduling infrastructure. The second part presents an actual planning and implementation framework.
Join Linux Journal's Mike Diehl and Pat Cameron of Help Systems.
Free to Linux Journal readers.View Now!
|The Firebird Project's Firebird Relational Database||Jul 29, 2016|
|Stunnel Security for Oracle||Jul 28, 2016|
|SUSE LLC's SUSE Manager||Jul 21, 2016|
|My +1 Sword of Productivity||Jul 20, 2016|
|Non-Linux FOSS: Caffeine!||Jul 19, 2016|
|Murat Yener and Onur Dundar's Expert Android Studio (Wrox)||Jul 18, 2016|
- Stunnel Security for Oracle
- The Firebird Project's Firebird Relational Database
- SUSE LLC's SUSE Manager
- Murat Yener and Onur Dundar's Expert Android Studio (Wrox)
- Managing Linux Using Puppet
- My +1 Sword of Productivity
- Non-Linux FOSS: Caffeine!
- Google's SwiftShader Released
- SuperTuxKart 0.9.2 Released
- Doing for User Space What We Did for Kernel Space
With all the industry talk about the benefits of Linux on Power and all the performance advantages offered by its open architecture, you may be considering a move in that direction. If you are thinking about analytics, big data and cloud computing, you would be right to evaluate Power. The idea of using commodity x86 hardware and replacing it every three years is an outdated cost model. It doesn’t consider the total cost of ownership, and it doesn’t consider the advantage of real processing power, high-availability and multithreading like a demon.
This ebook takes a look at some of the practical applications of the Linux on Power platform and ways you might bring all the performance power of this open architecture to bear for your organization. There are no smoke and mirrors here—just hard, cold, empirical evidence provided by independent sources. I also consider some innovative ways Linux on Power will be used in the future.Get the Guide