Crontabs

Introduction

Crontab is a list of commands that allows tasks (programs, scripts) to be run automatically at regular time intervals. For example, in RUTxxx routers it is responsible for executing such services as Automatic Reboot functions, SIM Idle Protection, Scheduled SMS and other functions and services that operate in a periodic manner.

The crontab can be opened for editing, adding, removing or modifying scheduled tasks. This article will provide an explanation of the crontab functionality principle and present some usage examples in hopes that it may help you configure your own crontab rules.

Crontab syntax and editing overview

This section of the guide overviews the syntax of crontab rules, editing methods and editing environment choices.

Editing environment

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Firstly, you must decide on what environment you're going to use for editing. Crontabs can be edited via a command line interface (CLI). RUTxxx routers offer a range of options in that regard. For example, you can use the CLI present in the router's WebUI (Services → CLI). Or you can login via SSH and make edits from there. The method of logging in via SSH is different on different systems: on Linux systems you can use the Terminal app login with the command ssh root@<routers_lan_ip_address>; on Windows systems you can use the free PuTTY client.

In all cases the login information is the same (user name: root; password: admin password (default: admin01)). The syntax of the commands is also the same, so feel free to follow the guide step-by-step whichever method you choose. You can find more detailed information on RUTxxx command line interfaces in our other wiki article Command line interfaces.

Once you've chosen your preferred environment, we can begin the overview of the syntax and editing of crontabs.

Syntax and editing

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Crontab entries are stored in the /etc/crontabs/root file. You can edit that file directly with the command vi /etc/crontabs/root, but it is more convenient to edit with crontab -e, which we'll be using for all examples in this guide. -e is an option of the crontab command; used for editing. Presented below is a list of all options that can be used with crontab:

Usage: crontab [-c DIR] [-u USER] [-ler]|[FILE]
                                                                          
       -c      Crontab directory                                            
       -u      User                                                         
       -l      List crontab                                                 
       -e      Edit crontab                                                 
       -r      Delete crontab                                               
       FILE    Replace crontab by FILE ('-': stdin)  

As a demonstration, lets try the -l option that shows the contents of the crontab file:

For the sake of this example, I've added a Ping Reboot rule to the router's configuration beforehand. In the figure above you can see that creating such a rule via the router's WebUI also generates an entry (encapsulated in green) in the crontab rule list. Let's examine this entry:

• /sbin/ping_reboot 1 8.8.8.8 NULL 2 56 5 1 0 cfg02c21d - indicates the action that is to be taken. In this case it is the /sbin/ping_reboot command with the options 1 8.8.8.8 NULL 2 56 5 1 0 cfg02c21d
• */5 * * * * - indicates the frequency at which the action is executed. It consists out of five segments: minute, hour, day of month, month and day of week in that order. A table of examples on how to specify this frequency is presented below:

Segment	minute	hour	day of month	month	day of week	Description
Possible values	0-59	0-23	1-31	1-12	0-6
Examples	*/5	*	*	*	*	every 5 minutes
	12	*/3	*	*	*	every 3 hours at 12 minutes
	57	11	15	1,6,12	*	At 11:57 Hrs on the 15th of Jan, June & Dec.
	25	6	*	*	5	At 6:25 AM every weekday (Mon-Fri)
	0	0	4,12,26	*	5	At midnight on the 4th, 12th and 26th of every month plus every Friday
	5,10	9,14	*	*	0,4	At 9:05AM, 9:10AM, 2:05PM and 2:10PM every Sunday and Thursday

There are several nuances to discuss here:

• a number indicates the exact time at which an action should occur. For example, if the number 5 was placed in the minute section, the specified action would be taken at the fifth minute of every hour.
• an asterisk (*) stands for every possible value of the relevant time unit. For example, if placed in the hour section, it would indicate that an action should be taken every hour.
• two numbers separated by a dash(-) indicate a range of numbers. For example, if 4-7 was placed in the month section, it would indicate than the specified action should take place every fourth, fifth, sixth and seventh month of a year.
• numbers separated by commas (,) indicate multiple specific time stamps at which an action should occur. For example, if 6,9,10 was placed in the hour section, the specified action would be taken at 6:00 AM, 9:00 AM and 10 AM every day. Ranges of numbers can also be separated by commas. For instance, 5-10,20-25 placed in the minute section would indicate that an action must occur every fifth through tenth minute as well as every twentieth through twenty fifth minute.
• an asterisk and a number separated by a forward slash (/) indicate a periodicity. For example, */10 placed in the minute section would indicate that the specified action should occur every 10 minutes.
• the day of week segment functions a bit differently than the others:
  • 0 indicates Sunday and 1-6 indicate Monday through Saturday.
  • since weekdays shift throughout the year, i.e., they are not dependent on specific days of a month, when they are used in conjunction with crontab, the action will be executed when the current time matches the value in either field. For instance, 5 5 5 * 5 would cause the specified command to be executed at 5:05 AM every fifth day of every month plus every Friday.

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To edit crontab, use crontab -e. This is analogous to using the vi command with the /etc/crontabs/root file, so the same rules apply here as when editing with vi. Type crontab -e, press "Enter" and a text editor for the crontab file will open. To start editing, press the "I" key on your keyboard; you can then edit the crontab file much like with a regular text editor (except you can't use your mouse pointer to place the text cursor anywhere you want; use the arrow keys on your keyboard instead). To save changes after editing, press the "Escape" ("Esc") button on your keyboard, type :x and press "Enter". To cancel changes and exit the editor, press "Control" (Ctrl) + "C" on your keyboard.

A simpler yet more restricted method of editing would be using the echo command to add lines to the crontab file. It can be used in such a manner: echo "Hello, world" >> /etc/crontabs/root. This command will add the text Hello, world to the crontab file, i.e., it adds whatever is specified within the quotation marks (" ") after the echo command to the file that is specified after the double more than (>>) symbol. For example, to add a rule that reboots the router everyday at midnight, we would have to use:

# echo "0 0 * * * reboot" >> /etc/crontabs/root

Using the command in such a way simply adds a new line with the denoted text at the end of the specified file. This is convenient when you just want to add a new rule(s) quickly, but unlike vi or crontab -e it doesn't offer any other editing capabilities. So if you want edit or remove existing rules, this method will not offer a solution and crontab -e should be used instead.

Examples

This section will provide some crontab usage examples in the hopes of helping you getting the hang of the system or even finding one that you could use. The examples provided here will be examined thoroughly but the editing method will not be discussed; you can read up on that in the section above.

Periodic SIM switch

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For this example we'll configure a rule that initiates a SIM switch every weekday at 6:45 PM. To execute a SIM card switch via CLI the command sim_switch is used, so we'll combined this with the crontab and configure the rule:

45 18 * * 1-5 sim_switch change

Let's overview what each segment indicates sequentially:

• 45 - the action must take place at minute 45 of the hour
• 18 - the action must take place at 6 PM
• * - all days of the month are applicable to the rule
• * - all months of the year are applicable to the rule
• 1-5 - the action must take place every Monday - Friday
• sim_switch - the command that will be executed
• change - option for the sim_switch command

To sum up, the first five segments denote the frequency of the sim_switch command. The option change specifies that the SIM card that is currently in use will be switched. So the entire entry will perform a SIM switch to the opposite SIM card every weekday at 6:45 PM.

Launching an OpenVPN server on specified hours

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For this example we'll configure a system that launches an OpenVPN server at the start of a workday (8 AM) and shuts down said server at the end of a workday (6 PM). For this we'll need to add two rules. Each rule will require multiple commands to be executed. Crontab can launch multiple commands the same way as a single command, only the && separator should be used to separate the different commands:

0 8 * * 1-5 uci set openvpn.7365727665725F64656D6F.enable=1 && uci commit && /etc/init.d/openvpn restart
0 18 * * 1-5 uci set openvpn.7365727665725F64656D6F.enable=0 && uci commit && /etc/init.d/openvpn restart

Let's overview what each segment indicates sequentially:

• 0/0 - the action must take place at minute 0 (just as the hour changes)
• 8/18 - the action must take place at 8 AM and 6 PM respectively
• */* - all days of the month are applicable to the rules
• */* - all months of the year are applicable to the rules
• 1-5 - the action must take place every Monday - Friday
• openvpn.7365727665725F64656D6F.enable=1/0 - enables/disables OpenVPN
• uci commit - commits configuration changes
• /etc/init.d/openvpn restart - restarts OpenVPN service
• && - used to separate different commands

The first line enables the OpenVPN server at 8 AM, the second disables it at 6 PM. In this example we used UCI to change the state of the OpenVPN server. You find more information on UCI here.

External links

https://www.chiark.greenend.org.uk/~sgtatham/putty/latest.html - PuTTY downloads page link