Cron is a Linux tool to run programs at specific times. Learn how to use cron with the Raspberry Pi to control the GPIO for periodic tasks.
- It would be really handy to have a way to run a program on a periodic basis, for example, once per day or once per week or very five minutes. Linux, and Raspbian, provides cron for just that reason. Let's take a look at how you use cron to trigger an event at a certain time or repeatedly. Hi, I'm Mark Niemann-Ross and welcome to this week's edition of Raspberry Pi Weekly. Every week, we explore the Raspberry Pi and share useful tips. Cron requires two things. An application to run and instructions for when to run it. For our demonstration, I've created a Python program that blinks an LED five times. Instructions for this very simple project are in the handout. You'll find the program in the exercise files as cron_blink.py. This is cron_blink.py and it doesn't matter where the program is located but you'll need a complete path name for it. Be careful about path names that include spaces, such as the space in Exercise Files. I've moved cron_blink.py to the Documents folder to avoid any spaces in the path name. To obtain the path name, I can right click on the icon and then go to Copy Paths. This will give me a complete path name to the application. To check the path name, I can open up a terminal window and use Edit, Paste. You can see that my cron_blink is located in the home directory /pi/Documents. Now, I can test that. I have to put in an application that's going to run the code. In this case, it's a Python 3 program so I type in python3 space and when I hit Return, that program should run. If it doesn't run here, it's not going to run in cron. Once you have a path name to an executable file, you'll need a time to run it. Cron is rich with ways to specify the time to run, which makes it confusing. I've found a very helpful website at crontab.guru. I'll show you where to put these commands in in just a minute but first, let's examine the command. To run a command every Monday at one 'clock in the afternoon, you'd type in 013, for one o'clock, star, star and a one and you can see at the very top line, crontab.guru has identified this as every one o'clock on Mondays. To run a command every five minutes, use slash. So for example, star /5 star, star, star, star runs a command every fifth minutes. Crontab.guru also supplies some random times as well as some examples. You'll notice that the smallest cron increment is for minutes, not seconds. Cron only checks for commands every minute so anything less, for example, every three seconds, would be pointless. When you have both a command and a cron time specifier, you're ready to enter that into crontab which is the table of cron entries. You can do that with crontab -e. Crontab will ask you which editor you want to use. In this case, I'm going to use nano, which is number one. I'm going to scroll down to the bottom of crontab and enter a command to run our cron_blink.py every five minutes. So in this case, I'll type in star /5 star space star space star space star, followed by python3, which is the command to run and then the application path. In this case, I'm going to paste in the path name to cron_blink.py. Running a program with cron happens entirely in the background and any text you print to screen won't appear. So you'll want a way to confirm your program is running. Do this by sending messages to a log file you can periodically check. In my case, I'm going to use the greater than sign and send it to /home/pi/mylogfile.txt. If you also want to capture error messages from the system, add two greater than sign, ampersand one after the command. This directs error messages into the first file. In this command, use of one greater than sign will overwrite the file each time. If you use two greater than signs, Linux will append commands to the file. This is great if you want to keep a running log but may create a huge file if you forget to occasionally delete the file. When you're finished, use Control + O to write the file out and Control + X to exit nano. Then reboot your Raspberry Pi. When your Raspberry Pi is back running, there are a couple of ways to confirm that is cron is actually working. Type in crontab -l. And that will list the current crontab. You can see that our command is at the bottom. Crontab only tells you if a job is scheduled, not if it ran successfully. For that, check your text file log for diagnostic messages. You might also find useful debugging tips in the syslog. Try using grep to filter through this rather large file. Sometimes cron jobs can be frustrating. And there is a well-written description of troubleshooting cron jobs at garyhall.org.uk. On a side note, several posts on the internet suggest using @reboot as a way of starting a command when the Raspberry Pi reboots and this is a bad idea. Number one, it won't work on shutdown or boot, only reboot. It may not work on some versions of Linux and you may have issues with root users versus just standard users. There are additional videos on using cron with Linux in this library. Check them out for in-depth discussions of cron and crontab. Thanks for joining me for this episode of Raspberry Pi Weekly. Be sure to join the LinkedIn group and check out previous episodes on LinkedIn Learning. I'll see you next week with more Raspberry Pi adventures.
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