Linux for Programmers #12 - Cronjobs

Key Takeaways

hello everybody and welcome to another linux for programmers tutorial video in this video i'm going to be covering cron jobs and the cron tab now a cron job is a job or a task or a script whatever you want to refer to it as that is scheduled to run at a certain time now you can schedule jobs to run every minute every five minutes every first day of the month you can schedule them at pretty much any time increment that you want and it's extremely useful to do so for example if you want to back something up every 24 hours or you want to send an email to someone every week whatever it may be you can use the cron tab and cron jobs to do that so with that said let's dive in after a quick word from the sponsor of this video and this series which is le note le note is one of the best companies for developers they make it easy to manage and scale your infrastructure with their intuitive cloud manager api cli and one click apps they also provide predictable pricing you always know how much you're going to be paying when you use le note not to mention their award-winning support there's no tiers no hands off just highly trained service professionals who actually answer the phone and solve your issue 24 hours a day seven days a week 365 days a year get started with lenode by hitting the link in the description and claim a hundred dollars in free credit when you sign up with a new account so with that said let's dive in to cron jobs so the first thing that we need to discuss when we're talking about cron jobs is our cron tap now a cron tab is specific to each user so every user on the system will have their own cron tab and this cron tab is kind of the manager of our cron jobs so if you want to view your cron tab you can type cron tab and then hyphen l and this will view the current cron tab now in our specific situation we don't have a cron tab for the root user because we've yet to create one so what we need to do is we need to make a cron tab so to do that you type cron tab and then hyphen e now hyphen e stands for edit so if you had an existing cron tab this would allow you to edit the existing cron tab if you don't have one then it will prompt you to create a new one and specifically when you run this command it will actually ask you what editor you want to view your cron tab in so you can pick nano you can pick vi you can pick emacs there's a bunch of other ones you can pick as well but choose whatever one you like now in my situation you're going to see that when i run this command it brings me into my cron tab and it's being edited in nano now it probably is going to prompt you to pick which editor you want to write or view your cron tab in so in that case you need to select what editor i would recommend nano it's just the simplest in my opinion and well that's what i'm using here so select your editor and then it should bring you into a tab that looks like this so this window right here is the cron tab and the cron tab allows us to manage our cron jobs so we can have multiple cron jobs that are all scheduled to run at different times or even the same time for that matter and we handle all of the scheduling and all the jobs that we want to run in the cron tab now i would recommend that you actually read through what is here because it's not that long and it gives you a pretty good idea of how this actually works but i'm going to skip through it and let you guys read this at your own pace so now what i'm going to do is show you how to set up a quran job so a current job again is just running something at a certain time increment so every 10 minutes every hour as i've said it's just some script or some command that you want to run at a certain point in time so let's say that you wanted to run like the ls command you actually could run that command every five minutes every 10 seconds every you know two hours whatever you can run it at whatever frequency you want you also could run a backup script for example at you know every day or every two hours or every sunday or every first day of the month you can set the scheduling to be as advanced as you want but any command that you want to run you can put inside of here and schedule it so it happens at a certain point in time now you also can have commands that run at boot so for example when you first boot the system you could have a command that runs at midnight that runs annually so once a year there's a ton of different things that you can do so i'm just trying to give you an idea of what's possible in here and let me show you a few examples of things that we can actually run so let's say that we want to run the ls command every one minute well what i would do to run that is i would type the following so i'm going to start with actually five asterisks here and then the ls command now the basic syntax for your cron job is you have five asterisks separated by spaces and then you have the command afterwards now you replace these asterisks here with what you need for scheduling so for example if i wanted to run this job here every five minutes or every one minute i forgot what i said let's go with one minute we would put slash one after the first asterisk now what this means is run this command ls every one minute so it doesn't matter what minute we're currently on but as soon as we set this command to run now every one minute this command will run because we put slash and then one so the asterisks represent the following going from left to right and i'll put this up on the screen i have an image i think that i can show you we have minute and then we have hour then we have day of month we have month and we have day of week now it is not mandatory to put numbers or values where all of the asterisks are in fact that's the reason why you have the asterisk is so that you can kind of have a placeholder if you don't want to actually use a specific increment of time so for example this is all we need to do to run the ls command every one minute since this first column here stands for minutes and you can kind of see right above it actually shows you what they stand for we just put slash standing for every and then one standing for the amount of minutes so one minute then we run this command every one minute so if i wanted to run a command every 15 minutes then what i would do is type asterisk slash 15 and then i would type four more asterisks and then the command so maybe i could just do something like cat now of course we know that's not really going to do anything for us but we can go ahead and run that command now we also can run scripts right and this is probably the more useful thing if we have a script that we've written we could run that command every hour every day whenever we want so anyways let me show you a few more things that we can do here for the the syntax of this chron job so if i wanted to run something not every 15 minutes but at every 15 minutes so when i say at every 15 minute i mean like 1 15 2 15 3 15 4 15 5 15 so on and so forth not every 15 minutes but at the 15th minute of every hour then what i would type is 15 and then for asterisks like that now i understand this is slightly confusing but when you want to schedule something every so at an increment of some time you do the asterisk you do the slash and then what that increment is if you want it to run at a specific time and in this case we want to run at every 15 minute of the hour we just write that time in the appropriate column so now we'll run at every 15th minute of every hour and then again i can put whatever command i want i'm just going to do placeholder command here because it doesn't matter what the command is i'm just trying to show you the syntax all right so now let's say that we actually wanted to run something at every 5th minute 10th minute and 15 minute of the hour so i want to run the same command multiple times throughout the hour then what i would do is the following i could do something like 5 10 and i don't think we need spaces 5 10 15 and then 4 asterisks and then my command this again means that we're going to run at 105 and 110 and 115 then 205 then 210 then 215. then 305 then 310 then 315 you get the idea it's the fifth 10th and 15th minute of every hour now that's pretty hard to pronounce but hopefully you get the idea so that is how that one works now let's say you wanted to execute something just every hour you want to do it every hour what you do is you would type zero and then you would put your four asterisks and then the command now this means run at the zeroth minute of every hour so you will have it happen every hour so we will continue in one second but i need to quickly thank the other sponsor of this video which is algo expert i'll go expert is the best platform to use when preparing for your software engineering coding interviews they have a data structures crash course a mock interviews feature and over 120 coding interview questions get started with algo expert today by hitting the link in the description and using the code tech with tim or a discount on the platform now let's say we wanted to run something every four hours now we're moving out of the minute column and we're going to move into the hour column so what i'm going to do is type the following i'm going to type 0 so not oh sorry 0 then i'm going to type asterisk slash four and then three more asterisks and then my command because this means we're in the hour column here by the way in the second asterisk that we are going to run every four hours at the zeroth minute of that hour so that's what this command tells us every four hours at the zeroth minute run this command that's as easy as it is now let's say we wanted to run something every day well then what we type would type is zero 0 and then 3 asterisks because the hour column is telling us the amount of hours that we want to run this at so this pretty much means run at midnight every single day run at the 0th minute and 0 hour of every day now let's say we didn't want this to run at midnight but we still want it to run every day we could set it to run at say 4 am or 5 a.m i could say 0 5 and then asterix asterix asterix this is in 24 hour time by the way and when i put this zero five this means run at the fifth hour of every day and the zeroth minute of that fifth hour so this means we're gonna run at five a.m every single day and then we can do a bunch of other ones as well so let's say now that i wanted to run something on the first day of every month well i would pick the time that i want to run this at so let's say i actually want to run this at something like 10 30. i put 30 then i could put 10 this means we want to run at 10 30 and i want to run this on the first day of every month so what i can do is i can put the day in the month that i want this to run at that's what dom stands for this third column so 30 10 1 and then two asterisks and then the command after this is going to run on the first day of every month at 10 30. so now let's say i actually wanted to run this every three months well if i want to run this at 10 30 on the first day of the month but every three months what i would do is the following i would say 30 10 i would say one assuming i wanted this to run on the first day of each month then i would say asterisk three and then one more asterisk and then my command now that's because we're moving into the fourth column now which is the month so when i say asterisk 3 this means that i want this to run every 3 months so hopefully that's clear but this is 10 30 on the first day of the month every three months now lastly we can also pick the day of the week that we want to run a command on so let's say we want to run a command every monday through friday so monday tuesday wednesday thursday friday but we don't want to run it on the weekends what i could do is the following i could type zero zero assuming we want this to run at midnight and then asterix asterix and then one hyphen five so now we've moved into the day of week column so the day of week column allows us to schedule tasks to run on certain days of the week so when i say one to five that means run this from monday to friday and the reason that works is because 0 represents sunday and 6 represents saturday so from 0 to 6 our valid integers here are valid numbers to put i also can put a range so 1 hyphen 5 which means run it in all of the days in between one and five now of course i could do one two three four five like that as well this is the same thing as doing one hyphen five but you are allowed to put a range and this just means run from monday to friday now the reason i have these two asterisks in between is because i don't care what day of the month it is i don't care what month it is i just care what time i'm running this at so i'm running this at midnight and i'm running this every monday to friday so you are allowed to skip asterisks when you write these commands out but you just have to understand what you're actually scheduling this for now most commonly you're going to want to run something every day or every hour or every few minutes you probably don't need to do something super complex like i'm showing you here but this again is just to give you an introduction to how the cron jobs work and the basic syntax so you probably will have to look up the syntax when you're doing something more specific in the future but hopefully this gives you a good idea so that was pretty much all i wanted to show you for the cron job syntax there's a few more things to show you though and these are kind of shortcuts for this scheduling syntax so let's say you don't want to deal with all this you don't want to have to figure out what all the asterisks mean well fortunately there's a few little tags that we can use so if i wanted to run something daily i could just type at daily which is just kind of a shortcut that you're allowed to use inside of here and then the command that i want to run afterwards now if i wanted to run something every day at midnight then what i would do is say midnight now this actually means the exact same thing as the daily command but it's just a bit more verbose because by default daily is going to run at midnight in fact almost all of these tags that i'm showing you by default will run at midnight if that's a valid option for the default all right now there's another one that we can use that's pretty useful this is reboot so if you want something to run no matter what when you start the machine or when it reboots then you would say at reboot and then whatever the script is that you want to run there's a few more you can probably guess what they are but we can do weekly we can do annually like that we can do yearly which is the exact same thing as annually again i don't know why they have multiple but you can use both of those they also have hourly and i think that's about it i believe there's monthly as well i don't think i showed that one so those are kind of the tags that you're allowed to use that are just shortcuts for this syntax right here yeah hopefully that gives you an idea of how cron jobs work now what i will show you as a last thing here is how you actually you know put your own scripts here because it's one thing to understand the syntax but how do i point to a script that i wrote and schedule that to run so i'm just going to exit out of this i'm not going to save this because i don't want all of that i'm going to show you that if i say ls here we have a dot sh file i have my test.sh so let's say i wanted to run this script every minute well what i would do is i would go inside of my crown tab so cron tab hyphen e and then inside of here i would point to that script so what i would say is let's go you know asterisk slash one asterix asterisk asterisk and then we're going to go slash root slash and then test dot sh and i'm just going to put a dot here because that's how we actually execute this command so now if i were to save this cron tab what would happen is this test.sh file would run every one minutes and that's all you need to know that's how this works you need the path to the file or to the script that you want to run and yeah you just put it here now of course you could write commands as well say you want to run a python script i'm not going to put some syntax for the cron job but you could say python or python3 and then whatever the path to the script is so maybe slash root slash script dot py and well that would go ahead and run the python3 command with that script so hopefully that is clear but that is how cron jobs work and that is the cron tab and that's all i have for this video so i hope you guys enjoyed if you did make sure to leave a like subscribe to the channel and i will see you in another youtube video [Music]