Crontab Tutorial

[ardchoille]

What Is Crontab?

A crontab is a simple text file that holds a list of commands that are to be run at specified times. These commands, and their related run times, are controlled by the cron daemon and are executed in the system's background. More information can be found by viewing the crontab's man page. We will run through a simple crontab example later.


How Does It Work?

The system maintains a crontab for each user on the system. In order to edit or create a crontab, you must use the text editor that is specified by your system. The nano text editor is the default text editor on my system. This text editor must be opened with the command crontab using the -e option. To create a crontab open a term and type:

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crontab -e

On the default system the user wil be given a choice of text editor. Each line represents a seperate crontab entry - also known as a "cron job". If you are not familiar with the nano text editor you should obtain a tutorial for it as that information is beyond the scope of this post.


Crontab Sections

Each of the sections is separated by a space, with the final section having one or more spaces in it. No spaces are allowed within Sections 1-5, only between them. Sections 1-5 are used to indicate when and how often you want the task to be executed. This is how a cron job is layed out:

minute (0-59), hour (0-23, 0 = midnight), day (1-31), month (1-12), weekday (0-6, 0 = Sunday), command

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01 04 1 1 1 /usr/bin/somedirectory/somecommand

The above example will run /usr/bin/somedirectory/somecommand at 4:01am on any Monday which falls on January 1st. An asterisk (*) can be used so that every instance (every hour, every weekday, every month, etc.) of a time period is used.

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01 04 * * * /usr/bin/somedirectory/somecommand

The above example will run /usr/bin/somedirectory/somecommand at 4:01am on every day of every month.

Comma-seperated values can be used to run more than one instance of a particular command within a time period. Dash-seperated values can be used to run a command continuously.

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01,31 04,05 1-15 1,6 * /usr/bin/somedirectory/somecommand

The above example will run /usr/bin/somedirectory/somecommand at 01 and 31 past the hours of 4:00am and 5:00am on the 1st through the 15th of every January and June.

The "/usr/bin/somedirectory/somecommand" text in the above examples indicates the task which will be run at the specified times. It is recommended that you use the full path to the desired commands as shown in the above examples. The crontab will begin running as soon as it is properly edited and saved.


Crontab Options

The -l option causes the current crontab to be displayed on standard output.
The -r option causes the current crontab to be removed.
The -e option is used to edit the current crontab using the editor specified by the VISUAL or EDITOR environment variables.

After you exit from the editor, the modified crontab will be checked for accuracy and, if there are no errors, installed automatically.


Crontab Example

Below is an example of how to setup a crontab to run updatedb, which updates the slocate database: Open a term and type "crontab -e" (without the double quotes) and press enter, type the following line, substituting the full path for the one shown below, into the editor:

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45 04 * * * /usr/bin/updatedb

Save your changes and exit the editor.

Crontab will let you know if you made any mistakes. The crontab will be installed and begin running if there are no errors. That's it. You now have a cron job setup to run updatedb, which updates the slocate database, every morning at 4:45.

Note: The double-ampersand (&&) can also be used in the "command" section to run multiple commands consecutively.

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45 04 * * * /usr/sbin/chkrootkit && /usr/bin/updatedb

The above example will run chkrootkit and updatedb at 4:45am daily - providing you have all listed apps installed.

I hope this little tutorial is of help to the community.

[overthetop]

This is a good tutorial for helping people learn about cron jobs. They are a very useful tool for every linux user.

[2013james]

There is now a useful crontab sandbox for trying out these examples at http://www.dataphyx.com/cronsandbox. Put in any combination of time/date values, get back a list of future run-times.

[Boca]

Hi, I have used crontab on other pc and am now setting up on a new 18.1 Mate but it acts differently...
on other pc ( 17.3 Cinnamon) I would get a "help file" with examples.

Do I need to install something?

Please see system config info with my crontab attempt

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tony@tony-Latitude-E6430 ~ $ inxi -Fxz
System:    Host: tony-Latitude-E6430 Kernel: 4.4.0-71-generic x86_64 (64 bit gcc: 5.4.0)
           Desktop: MATE 1.16.1 (Gtk 3.18.9-1ubuntu3.2)
           Distro: Linux Mint 18.1 Serena
Machine:   System: Dell (portable) product: Latitude E6430 v: 01
           Mobo: Dell model: 0CPWYR v: A00 Bios: Dell v: A07 date: 10/08/2012
CPU:       Dual core Intel Core i5-3210M (-HT-MCP-) cache: 3072 KB
           flags: (lm nx sse sse2 sse3 sse4_1 sse4_2 ssse3 vmx) bmips: 9967
           clock speeds: max: 3100 MHz 1: 1230 MHz 2: 1486 MHz 3: 1372 MHz
           4: 1299 MHz
Graphics:  Card: Intel 3rd Gen Core processor Graphics Controller
           bus-ID: 00:02.0
           Display Server: X.Org 1.18.4 drivers: intel (unloaded: fbdev,vesa)
           Resolution: 1366x768@60.01hz
           GLX Renderer: Mesa DRI Intel Ivybridge Mobile
           GLX Version: 3.0 Mesa 11.2.0 Direct Rendering: Yes
Audio:     Card Intel 7 Series/C210 Series Family High Definition Audio Controller
           driver: snd_hda_intel bus-ID: 00:1b.0
           Sound: Advanced Linux Sound Architecture v: k4.4.0-71-generic
Network:   Card-1: Intel 82579LM Gigabit Network Connection
           driver: e1000e v: 3.2.6-k port: f080 bus-ID: 00:19.0
           IF: eno1 state: up speed: 1000 Mbps duplex: full mac: <filter>
           Card-2: Intel Centrino Ultimate-N 6300
           driver: iwlwifi bus-ID: 02:00.0
           IF: wlp2s0 state: down mac: <filter>
Drives:    HDD Total Size: 1338.4GB (52.1% used)
           ID-1: /dev/sda model: TOSHIBA_MK3261GS size: 320.1GB
           ID-2: USB /dev/sdb model: My_Passport_07A8 size: 1000.2GB
           ID-3: USB /dev/sdc model: Flash_Disk size: 2.0GB
           ID-4: USB /dev/sdd model: Flash_Reader size: 16.2GB
Partition: ID-1: / size: 290G used: 14G (6%) fs: ext4 dev: /dev/sda1
           ID-2: swap-1 size: 4.16GB used: 0.22GB (5%) fs: swap dev: /dev/sda5
RAID:      No RAID devices: /proc/mdstat, md_mod kernel module present
Sensors:   System Temperatures: cpu: 47.0C mobo: N/A
           Fan Speeds (in rpm): cpu: N/A
Info:      Processes: 206 Uptime: 17:45 Memory: 2873.8/3825.3MB
           Init: systemd runlevel: 5 Gcc sys: 5.4.0
           Client: Shell (bash 4.3.461) inxi: 2.2.35 

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tony@tony-Latitude-E6430 ~ $ crontab -e
no crontab for tony - using an empty one
888

?

Thanks, Tony

[Habitual]

Tutorial is incomplete, IMO until you cover environments (or the lack of them) in cron