UNIX Scheduling with cron

scottalanmiller

In most UNIX systems (definitely including all major Linux Distributinos, the BSD family and Solaris family) the main system task scheduling system is the cron dæmon and utility, named after Chronos, the Greek god of time. Cron is a robust scheduling system that can be used by any user on the system.

Cron is a dæmon that reads a set of simple text files to determine what scheduled tasks it is to execute and when.

While it is possible to edit cron's text files directly, this is discouraged. Under most circumstances, we instead manage cron through the crontab command line utility. This tool works just like using vi manually does but automatically manages the files on our behalf so that we do not need to now where they are, handles user management and has a syntax checker so that we are much more likely to avoid simple mistakes in our cron entries.

The cron files managed by crontab are located at /var/spool/cron/crontabs on most systems, but they are not meant to be edited manually.

There are just two common options when using the crontab command, crontab -l will list the cron table of the current user and crontab -e will open the crontab editor. If you need to remove a cron table completely for the user, it is crontab -r.

In nearly all cases, you will start without any scheduled tasks. We can verify like this:

$ crontab -l
no crontab for scott

If we run crontab -e we can create a cron table and add in some entries. Cron is an incredibly simple system to use. However, the complexity with it comes in the format of its scheduling syntax.

The cron table format is pretty easy to use, once you get used to it, but the fields can be difficult to remember so it is common and expected that you will normally look them up when making an entry unless you do this very commonly.

The cron table is based on a "column" system. Here is the format:

min hour day(month) month day(week) command

And we fill in each of the first four fields with a single number, a list of comma delimited numbers or a hyphen denoted range of numbers and the final field is the command that we want to run, exactly as we would run it normally. Each numeric field will take an asterisk (*) which is the standard wildcard meaning "any".

Min: The minute of the hour, just like on the clock.
Hour: The hour of the day in 24 hour format.
Day of the Month: The calendar number.
Month: The month of the year.
Day of the Week: 0 is Sunday, so is 7

The fields can be in these forms:

Single Number: 5
List of Numbers: 0,15,30,45
Range of Numbers: 1-5
All: *

So let's say that we want to run a custom script that we have written that emails out a notification every day at five in the afternoon that tells everyone that it is time to go home. We would do it like this:

0 17 * * * /opt/scripts/gohomeremindermail.sh

The zero is the minutes of the hour and the 17 is 5PM on a 24 hour clock. The next three asterisks denote that this is to go out every day of the month, every month of the year, any day of the week.

Now wait a minute, what if we want to limit that to week day? Easy enough, try this:

0 17 * * 1-5 /opt/scripts/gohomeremindermail.sh

You add one line for each item that you wish to schedule. Each item is known as a "cron job". The cron jobs in your personal cron table will run with your permissions; they will act as if they were run by you. Cron scheduling is actually very simple and straight forward.

There are some special case items that you can put into the cron table that do not exactly follow the above format. The @reboot directive tells cron to execute the cron job immediately following a system reboot, but at no other time. This can be very handy for special case jobs.

The format looks like this:

@reboot /home/scott/mystuff/mycoolscript.sh

If you have the necessary permissions, like if you are the root user, you can use the crontab command to modify the cron tables of any user as well by adding a -u and the user's name like this:

crontab -e -u scott

Advanced Crontab Entries with Fractions

One quirk of the cron table format is that it allows the use of fractions for the numbers. Not exactly fractions but an "every" variable. So in this example...

0-30/5 * * * * /opt/apps/sendanalert

The sendanalert program would fire every five minutes from the start of every hour until the half, then wait half an hour. It's a short hard to writing 0,5,10,15,20,25,30

*/10 * * * * /opt/apps/sendanalert

This one would have send the alert every ten minutes.

Also available are some other handy short hands that are rarely used...

@yearly
@weekly
@daily

Part of a series on Linux Systems Administration by Scott Alan Miller

JaredBusch

Thanks,
I always have to look this up again because i never do it often enough to have ti 100% in memory.

scottalanmiller

@JaredBusch said in UNIX Scheduling with cron:

Thanks,
I always have to look this up again because i never do it often enough to have ti 100% in memory.

Definitely check it out and let me know if anything is unclear or missing. Cron is always just a little confusing and I wanted to balance making it easy but having the needed info.

JaredBusch

@scottalanmiller said in UNIX Scheduling with cron:

The fields can be in these forms:

Single Number: 5

List of Numbers: 0,15,30,45

Range of Numbers: 1-5

All: *

From memory here, not bothering to check Google at the moment, but can you not also mix a list and range?

0,8-17,22

So that would send at midnight, and every hour 8-17 and then 22?

stacksofplates

Just to point out, there are some other strings you can use also.

@yearly
@weekly
@daily

You can also use /

0-30/5 * * * *

Is every 5 minutes for the first 30 minutes.

scottalanmiller

@JaredBusch said in UNIX Scheduling with cron:

@scottalanmiller said in UNIX Scheduling with cron:

The fields can be in these forms:

Single Number: 5

List of Numbers: 0,15,30,45

Range of Numbers: 1-5

All: *

From memory here, not bothering to check Google at the moment, but can you not also mix a list and range?
0,8-17,22
So that would send at midnight, and every hour 8-17 and then 22?

Yes, I'm pretty sure that that is correct.

tonyshowoff

I've been dealing with cron for... 22 years I guess, and it still irritates the hell out of me. Anyway, great work.

coliver

@johnhooks said in UNIX Scheduling with cron:

Just to point out, there are some other strings you can use also.
@yearly
@weekly
@daily
You can also use /
0-30/5 * * * *
Is every 5 minutes for the first 30 minutes.

I didn't know about the '/' That's really good to know.

scottalanmiller

@tonyshowoff said in UNIX Scheduling with cron:

I've been dealing with cron for... 22 years I guess, and it still irritates the hell out of me. Anyway, great work.

Thanks.

scottalanmiller

@coliver said in UNIX Scheduling with cron:

I didn't know about the '/' That's really good to know.

I tacked it on in the advanced section at the end. This one I've seen used in real life. Never seen anyone use @monthly, for example. I know that it exists, but just have never seen it get used.

tonyshowoff

@scottalanmiller said in UNIX Scheduling with cron:

@coliver said in UNIX Scheduling with cron:

I didn't know about the '/' That's really good to know.

I tacked it on in the advanced section at the end. This one I've seen used in real life. Never seen anyone use @monthly, for example. I know that it exists, but just have never seen it get used.

I do, for expiring old user sessions.

travisdh1

I was going to say that it's another handy reference to the time formatting for cron, but I normally just use the man page.

Alex Sage

6:03 AM

03 6 * * * <command>

or

3 6 * * * <command>

???

JaredBusch

@aaronstuder said in UNIX Scheduling with cron:

6:03 AM

03 6 * * * <command>

or

3 6 * * * <command>

???

The latter.

Alex Sage

Thanks!