Template:FAQ:Agent 07

'exec' will run the specified command and return the exit status and output. Any arguments are passed directly to the command, with no special interpretation.

'sh' is similar, but invokes a shell to run the command line given. This means that quoted arguments will be recognised as such, and also allows redirection, and other similar shell interpretation. The results are returned in exactly the same way.

'extend' is also similar, but provides a richer and more flexible MIB framework - both for configuring the exact command to be run, and for displaying the results.

None of these mechanisms require the command to have any knowledge of SNMP, or the fact that they are being used in this manner. But the output is returned in a fixed format, and it is up to the receiving application to interpret this appropriately.

Note that the "relocatable" form of the 'exec' directive (exec OID ....) produces MIB output that is not strictly valid. For this reason, support for this has been deprecated in favour of extend OID ... , which produces well-formed MIB results (as well as providing fuller functionality).
The most recent releases of the agent don't include support for "relocatable exec" by default. This needs to be explicitly included when the agent is first compiled, by including the module ucd-snmp/extensible instead of agent/extend.

'pass' is a more general mechanism for implementing arbitrary MIB objects. The specified command will be invoked for any request within the named MIB subtree, and passed details of the requested OID. It should return the information relevant to the requested OID.

'pass-persist' is similar, but the command will continue running even after the initial request has been answered. These two mechanisms can be used to implement a particular MIB, following the correct MIB structure (as opposed to the fixed format of exec/sh/extend).

All of these mechanisms are described in the 'snmpd.conf(5)' man page, in the section entitled Extending Agent Functionality.