How to schedule tasks on Linux
Nice! But how can I use it? What are specific use cases?
Or why should I even care about it?
Well, every company I worked before had its routines. Daily, weekly, monthly, on even non-periodic. Most of them were simple tasks you could easily manage through Google Calendar, Wunderlist, Trello, Slack, or whatever, just choose your weapon.
In some cases though, these weapons were not sufficient. Sometimes you have to access your database and check if a stock price raised above an specific value, or monitor if an e-mail message was received and extract its attachment which is a zip file with a password including a spreadsheet with some useful data you need to import to your database and… you get it.
In theeese cases you’ll think about replacing your hummer with a bazooka.
Before getting started
You’ll need to run a Linux terminal. If you’re on Windows and still want to try it, you can use a free cloud solution such as Cloud9 IDE. Go ahead! I’ll wait.
Sign up, log in to your account, and find Create a new workspace.
Fill in the form, choose Private and Custom…
Click Create workspace, wait for it to load, and…
We’re good to go!
Create your first scheduled task
- On the terminal window, type crontab -e to edit your crontab file
At your first time, Linux will ask you to select an editor, just like below.
- I am a Vim user, so I’d type 3 and press <Enter>
It will open up a black screen. That’s your text editor. Don’t be afraid.
- Type i to enter insert mode on Vim.
- Type the following line of code:
* * * * * date >> ~/workspace/log.txt
It is saying: add the current date to the log.txt file every minute.
Of course you can use another folder or filename. Go ahead and change it.
- Press <Escape> to leave insert mode
- Finally, type :wq and press <Enter> to quit Vim
Note that w stands for write (or save), and q for quit
- You’ll see the crontab: installing new crontab message,
It means your new crontab file was successfully loaded into memory.
Make it run
Let’s check if your cron process is running:
- Type pidof cron
- If it outputs nothing, try using sudo cron start
- Now you should see its process id (pid)
It means it is working! Check your log.txt file by tailing it:
- tail -f ~/workspace/log.txt (or just tail -f log.txt)
Beautiful, isn’t it? Now let’s learn how to stop it.
Stop your tasks
You can either edit your crontab, deleting the corresponding line, or just hard-kill the running cron process. Let’s see the second method.
- Start by typing sudo kill `pidof cron`
- sudo gives you super powers, meaning super user do
- kill will stop the process given as argument
- pidof cron gets the id of the cron process
Quick tips on Linux processes
To view a dynamic real-time list of running processes, use the top command. Then press q to quit, or h for help. Note the cron process at the bottom.
Otherwise, you can just simply use ps -A command.
And a few more tips:
- to get your cron PID in one single command, try pidof cron
- to kill your cron job, use sudo kill 19604
- or, altogether, you could just type sudo kill `pidof cron`
Now let’s get going!
That’s quite the same as before:
- Just open it by typing crontab -e
- Now you’re using Vim (or your default text editor)
- Type i to insert mode
- Edit the file as needed
- Then <Escape> :wq <Enter>
More info on crontabs
You can always use the man linux command by typing man cron into your terminal. It will show you a detailed manual on how this crontab works.
To change how frequent your tasks are schedule, you need to have a brief understanding of how the first 5 parameters work. Let’s take a look.
So there are five time arguments, a user, and a command.
- Minute: use * for every minute, or type the minute you want.
You can also write multiple comma-separated minutes.
And also say every five minutes, e.g., by using */5.
- Hour, Day of Month, Month, Day of Week, follow the same logic.
- User: might be root, for administrative purposes, or your own user.
- Command: just use the command you’d type to your terminal.
Otherwise, if you’re in a hurry (or just lazy!), check out the link below:
http://crontab-generator.org/ — it’s a simple site to help you build a crontab.
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