# 11 External checks

#### Overview

External check is a check executed by Zabbix server by running a shell
script or a binary. However, when hosts are monitored by a Zabbix proxy,
the external checks are executed by the proxy.

External checks do not require any agent running on a host being
monitored.

The syntax of the item key is:

    script[<parameter1>,<parameter2>,...]

Where:

|ARGUMENT|DEFINITION|
|--------|----------|
|**script**|Name of a shell script or a binary.|
|**parameter(s)**|Optional command line parameters.|

If you don't want to pass any parameters to the script you may use:

    script[] or
    script

Zabbix server will look in the directory defined as the location for
external scripts (parameter 'ExternalScripts' in [Zabbix server
configuration file](/manual/appendix/config/zabbix_server)) and execute
the command. The command will be executed as the user Zabbix server runs
as, so any access permissions or environment variables should be handled
in a wrapper script, if necessary, and permissions on the command should
allow that user to execute it. Only commands in the specified directory
are available for execution.

::: notewarning
Do not overuse external checks! As each script
requires starting a fork process by Zabbix server, running many scripts
can decrease Zabbix performance a lot.
:::

#### Usage example

Executing the script **check\_oracle.sh** with the first parameters
"-h". The second parameter will be replaced by IP address or DNS name,
depending on the selection in the host properties.

    check_oracle.sh["-h","{HOST.CONN}"]

Assuming host is configured to use IP address, Zabbix will execute:

    check_oracle.sh "-h" "192.168.1.4"

#### External check result

The return value of the check is standard output together with standard
error (the full output with trimmed trailing whitespace is returned
since Zabbix 2.0).

::: noteimportant
A text (character, log or text type of
information) item will not become unsupported in case of standard error
output.
:::

In case the requested script is not found or Zabbix server has no
permissions to execute it, item will become unsupported and
corresponding error message will be set. In case of a timeout, the item
will be marked as unsupported as well, an according error message will
be displayed and the forked process for the script will be killed.