Checkmk on the command line

When administering Checkmk, with a few exceptions you need never work as the root-user. In this article we will generally assume that you are logged in as a site user. That is done with, e.g.:

It is also possible to make a direct SSH-login to a site without a detour via root. Since the site user is a ‘completely normal’ Linux user, you must simply assign a password for this (which requires root-permissions, once only, for the configuration):

Afterwards an SSH-login directly from another computer should be possible (Windows-users preferably use PuTTY for this). From Linux this login is simply performed in the command line using the ssh command:

At the first login a ‘warning’ regarding an unknown host key will probably be received. When you are certain that in this brief moment no attacker has taken over your operating system’s IP-address, you can simply verify it with yes.

You can also work with the command line on the Checkmk-appliance. How that is done is explained in its own article.

So that as few problems as possible arise, particularly as a result of individual distributions or differing operating system configurations, the Checkmk-system ensures that the site user – and likewise all of the monitoring’s processes – always have a clearly defined environment. Along with the home directory and the permissions, the environment variables play an important role.

Among other things, when logging in as a site user the following variables will be set or modified. These variables are available for use in all processes running within the site. This also applies to scripts that are indirectly invoked by these processes (for example, a user’s own notification scripts).

With the env command you can output all of the environment variables – adding | sort to this command arranges the list a bit more clearly:

Under Linux the environment is an attribute of a process. Every process has its own variables, which it automatically passes on to sub-processes. These start initially with the same, inherited variables, but can also alter them.

With the env command you can always only view the current shell’s environment. If you suspect there is an error in a particular process’s environment, with a small trick you can nonetheless output a listing of its environment. For this you only need the process-ID (PID). You can identify this with, e.g., ps ax, pstree -p or top. With this you can then access the process’s environ file directly via the /proc file system. Here as an example is a suitable command for the PID 13222:

If you require custom variables for your own scripts or other software to be run in the site, store them in the etc/environment file which has been specially created for this purpose. All variables defined here will be available everywhere within the site:

If you wish to customize your shell (Prompt or other things), you can perform this as usual in the .bashrc file. Environment variables nonetheless belong to etc/environment, so that they are certain to be available to all processes.

There is also nothing to prevent you having your own .vimrc file if you like working with VIM.

The following graphic shows the most important directories in a Checkmk installation with a site named mysite and a <version> called for example 2.0.0p8.cee:

The basis for this structure is provided by the /omd directory. Without exception, all of the files for Checkmk are found here. /omd is in fact a symbolic link to /opt/omd, while the actual physical data is located in /opt – but all data paths in Checkmk always use /omd.

Important is the separation of data (highlighted yellow) and software (blue). The site’s data is found in /omd/sites, and the installed software in /omd/versions.

Like every Linux user, the site user also has a home directory, which we refer to as the site directory. If your site is named mysite it will be found in /omd/sites/mysite. As usual in Linux the shell abbreviates the its own home directory with a tilde (~) (or swung dash). Since immediately following a login you will actually be in this directory, the tilde appears automatically in the input prompt:

Subdirectories of the site directory are shown relative to the tilde:

A number of subdirectories are located within the site directory, these can be listed with ll:

As can be seen, the directories bin, include, lib, share and version are symbolic links. The rest are ‘normal’ directories. This mirrors the separation of software and and data as explained above. The software directory must be accessible as a subdirectory in the site, but it is physically located in /omd/versions, and can also possibly be used by other sites.

The software directories, as usual under Linux, belong to root and thus may not be altered by a site user. The following subdirectories are present – those in the example are physically located within the /omd/versions/2.0.0p8.cee, and they are accessible via symbolic links from the site directory:

The version/ symbolic link is a ‘intermediate stop’ and serves as a relay point for the version used by the site. During a software update this will be switched from the old to the new version. Nonetheless, please do not attempt to perform an update manually by altering the link, since an update requires a number of other further steps – which will fail.

The actual data for a site is found in the remaining subdirectories in the site directory. Without exception, these belong to the site user. The site directory itself is also included. Checkmk stores nothing apart from the directories listed there. You can create your own files and directories without problem here, in which tests, downloaded data, copies of log files, etc. can be kept as desired.

The following directories have been predefined:

As just shown in the above table, the local directory with its numerous subdirectories is intended for your own extensions. In a new site, all of the directories in local/ are initially empty.

With the practical tree command you can quickly get an overview of the structure of local. The -L 3 option restricts the depth to 3:

All of the directories in the lowest level are actively integrated in the software. A file stored here will be treated in the same way as if it was in the directory with the same name within /omd/versions/…​ (or respectively, in the logical path from the site under bin, lib or share).

Example: In the site, executable programs will be searched for in bin and in local/bin.

Here it applies that in the case of identical names the file in local always has priority. This enables modification of the software without the need to change installation files in /omd/versions/. The procedure is simple:

Regarding point 3 above, if it is not known exactly which service to which the change applies, simply restart the complete site with omd restart.

In Checkmk – as already-described – the log files are stored in the directory var/. All log files of the relevant components can be found there:

On the web interface you can easily configure the extent to which data should be written to the log files by searching in Setup > General > Global settings for all entries with logging:

Alternatively it is possible to also customize the log levels on the command line in configuration files. The files are each called global.mk, but are located in different directories. Specify the entries if they are not already present, which is the case, if a Factory setting has not yet been changed.

In this file the Monitoring Core, Notifications and Alert Handlers entries are set:

In this file you can set the User Interface logging.

The amount of logged data increases inversely as the count decreases. The lowest log level is 50 (Critical) while the most data will be logged at 10 which corresponds to the highest (Debug).

This file is used for Livestatus Proxy logging. The possible values here correspond to those for User Interface logging.

Important: Log files can quickly become very large if a high level has been set. It is generally advisable to use such settings for a 'temporary' customization, as an aid in problem identification for example.

The command cmk is actually an abbreviation for check_mk, introduced to make the command faster to type. The command includes a built-in, very detailed online help, that can be called up with --help option:

As you can see in the command above, we have called help with the option -h instead of --help. Because what is true for the command itself is also true for its options: The less there is to type, the faster it goes. Not for all options, but for those that are often needed, there is therefore a short form in addition to the long form. Even though the long form is more intuitive, especially for beginners (check_mk --list-hosts) than the short form (cmk -l), we will use the short form in the User guide. If in doubt, you can always consult the command help. A longer look in the command help is a good idea in any case, as we will not present all options in the User guide.

By entering an option, you start the command cmk in a certain mode. Here follows the overview of the options we will present in this chapter, but also in other parts of the manual:

In some modes, further, specific options are available to you, e.g. you can limit the service discovery to certain checks, e.g. to the check df with the command cmk -I --detect-plugins=df myserver123.

A number of options always work — regardless of the mode with which the command is executed:

In the following section we will show how the commands can be used. The examples are mostly shown in an abbreviated form.

The commercial editions utilizes the Checkmk Micro Core (CMC) as its monitoring core, Checkmk Raw uses Nagios. An important task for the cmk is the generation of a configuration file that is readable for the core, and which contains all of the configured hosts, services, contacts, contact groups, time periods, etc. On the basis of this information the core knows which checks are to be executed and which objects it should provide using the GUI’s Livestatus.

For Nagios as well as for the CMC, it is fundamental that the number of hosts, services and other objects always remains static during the operation, and that this number can only be altered through the generation of a new configuration, followed by a reloading of the core. With Nagios a restart of the core is also needed. The CMC has a very efficient function for the reloading of its configuration during active processing.

The following table highlights important differences between the configurations of both cores:

Regenerating the configuration is always necessary if the contents of the configuration file in etc/check_mk/conf.d, or automatically-detected services in var/check_mk/autochecks have been modified. The Setup keeps a record of such changes and highlights them in the GUI as to activating changes. Should you ‘bypass the Setup’ by modifying the configuration manually or with a script, you will also need to attend to the activation manually. The following commands serve this function:

To summarize: If you want to customize a Checkmk configuration and activate the changes, in Nagios you will subsequently require:

And with the CMC:

A second mode in Checkmk deals with the execution of a host’s Checkmk-based checks. With this you can allow all automatically detected, and also manually configured services, to be immediately checked, without needing to bother yourself with the monitoring core or the GUI.

To do this, enter cmk --check followed by the name of a host configured in the monitoring. Since the --check option is the default option of cmk, you can also omit it. In addition, you should always add the two options -n (do not send results to the core) and -v (output all results). More on this in the description of the options below.

Further tips:

The following options influence the command:

The command cmk -d retrieves and displays the output from a host’s Checkmk agent. With cmk -d agent data is retrieved in the same way as with the monitoring. It is neither validated nor processed. Thus the data showed exactly matches the data that is turned over to the Agent Controller (when TLS encryption is enabled) or to a tunneling program in case datasource programs are configured.

You can even run cmk -d using the name or IP address of a host that is not configured in the monitoring. In this case legacy settings for the host will be assumed (TCP connection to port 6556, no Agent Controller, no encryption, no datasource program).

In the commercial editions you can also bake the agents from the command line, as you would otherwise do via the web interface. This gives you the option, for example, of updating the agents regularly, for example via a cronjob.

To bake the agents, use the option -A followed by the name of a host (or several):

The output shows that the agent packages available for the host myserver123 have been successfully baked. If you do not specify a host, the packages will be baked for all hosts.

The command only bakes when necessary. If the packages are still up to date, the output will look something like this:

You can still force baking with the -f (force) option.

The cmk -l command simply lists the names of all hosts configured in the Setup:

Because the data is provided ‘naked’ and ‘unprocessed’, it is easy to use in scripts – for example a loop across all host names can be constructed:

If, instead of echo you insert a command that performs something meaningful, this can be really useful.

The cmk --list-tag invocation likewise outputs host names, but also offers the possibility of filtering by host tags. Simply enter a host tag and you will receive all hosts having this tag. The following example lists all hosts that are monitored by SNMP:

Enter multiple tags and they will be linked with ‘and’. The below command delivers all hosts that are monitored by both SNMP and Checkmk agents. As no hosts fulfill this condition, the output remains empty:

For one or more specified host, cmk -D displays the configured services, host tags, labels and other attributes. Because the list of services is so extensive it can look somewhat confusing on the terminal. Send the output through less -S to avoid a break:

Checkmk provides a large number of ready to use plug-ins as standard. In every release a few new ones are added, and Version 2.4.0 already includes more than 2,000 plug-ins. Three cmk command options give you access to information about these plug-ins.

cmk -L produces a table of all plug-ins with their name, type and a description. At the same time, any self-written plug-ins stored in local/ will also be listed.

The following are possible types:

The list can of course be filtered simply with grep if something specific is being searched for:

If you want more information on a certain plug-in, documentation as manual page (or man page) can be called up with cmk -M:

This produces the following output:

Using cmk -m with no further options will access a complete catalog of all check plug-ins man pages.

You can navigate interactively in this catalog:

If you are expecting a comprehensive compendium covering the exact structure of all of the configuration files used by Checkmk, we will unfortunately have to disappoint you here. The complexity and diversity contained in the configuration files is simply too much to describe completely in this User guide.

The following example shows an entire completed parameter set for the check plug-in which monitors file systems in Checkmk. Because of the many parameters, the screenshot is divided into two parts, and set in lower-case characters:

The corresponding passage in the configuration file looks like this (somewhat more nicely formatted):

As can be seen, there are more than 10 different parameters, each with its own individual logic. Some are configured using floating-point numbers, (0.8), some with integers (24), some with keywords ('onlow'), some with boolean values (True), and others using tuples to code various combinations of these ((5.0, 10.0)).

This is just one example from over 2,000 plug-ins. And there are of course other configurations possible as check parameters: One only needs to think of time periods, Event Console rules, user profiles, and many more.

Of course that doesn’t mean you cannot use text files as configuration! If you don’t yet know the exact syntax for your chosen configuration task, you only need the correct tool for it – and this tool we call Setup:

You thus only need to know in which file Setup writes.

Note: When it comes to the names of directories, files or even file contents, you will often find the term wato. WATO is the abbreviation for Web Administration Tool: the Checkmk configuration tool up to and including version 1.6.0. The function of WATO has been taken over by the Setup menu, or Setup for short, from version 2.0.0 onwards. Although WATO has been (almost) completely replaced by Setup in the web interface, it lives on in the file system.

There is a practical command for finding out which file Setup has just changed: find. By invoking ‘find’ with the following parameters you can find all files (-type f) under etc/ which have been altered within the last minute (-mmin -1):

The basis of a configuration is always the etc/check_mk directory. Below this is a subdivision into various domains, which generally apply to a specific service. At the same time each has a directory with the suffix .d, under which all files with the suffix .mk will be read automatically in alphabetic order. In some there will also be a main file which is read first of all. This is intended only for manual alteration, and is never modified by Setup.

Below the .d/ configuration directories there is always the subdirectory wato, e.g. etc/check_mk/conf.d/wato. The Setup fundamentally only reads and writes here. However, the service responsible for the configuration directory also reads the other files in "its" .d directory, if you have stored some manually-created files there. This means:

In purely formal terms, all of Checkmk’s configuration files are written in Python 3 syntax. There are two types of files:

The executable files can be recognized by their having variables which are substituted for assignments with values (=). The other files usually contain a Python dictionary which begin with an opening bracket {. Sometimes they are simple values.

If a non-ASCII character is required in a file (a German Umlaut (ä, ö, ü), for example), the following comment must be coded in the first or second line:

A syntax error will otherwise occur when reading the file. For further tips on Python syntax we recommend visiting a specialist site, for example: The Python Language Reference.

If you edit configuration files in etc/check_mk/ manually, it is a good idea to have the configuration checked. You can do this with the tool cmk-update-config, which is actually only executed automatically after a version update with omd update:

For example, in the excerpt above you can see the reference to an unclosed bracket.