Query if nbdkit is installed

 nbdkit --version

This will fail with an error and non-zero exit code if nbdkit is not installed or not working.

Query basic configuration

 nbdkit --dump-config

lists information about how nbdkit was configured. The most important fields in the output are the name of the directory where nbdkit looks for plugins and the version of nbdkit, eg:

 plugindir=/usr/lib64/nbdkit/plugins
 version=1.20.1
 version_major=1
 version_minor=20

Test nbdkit ≥ version

 $ nbdkit --dump-config | grep ^version_minor
 version_minor=20
 $ major=$( nbdkit --dump-config | grep ^version_major | cut -d= -f2 )
 $ minor=$( nbdkit --dump-config | grep ^version_minor | cut -d= -f2 )
 $ if [ $major -eq 1 ] && [ $minor -lt 12 ]
   then echo 'nbdkit >= 1.12 is required'; exit 1; fi

These fields were first added in nbdkit 1.16.5 and were not present in earlier versions.

Query information about a particular plugin

 nbdkit pluginname --dump-plugin

(where pluginname is the name or full path of a plugin) will dump information about that plugin, eg:

 $ nbdkit file --dump-plugin
 path=/usr/lib64/nbdkit/plugins/nbdkit-file-plugin.so
 name=file
 version=1.20.1
 api_version=1
 struct_size=176
 thread_model=serialize_requests
 [etc]

Plugins which ship with nbdkit usually have the same version as the corresponding nbdkit binary. The nbdkit binary will always be able to utilize plugins compiled against an older version of the header; however, newer plugins may not be fully supported by an older nbdkit binary (for example, a plugin compiled with "NBDKIT_API_VERSION" of 2 fails to load with an older nbdkit that only knows "NBDKIT_API_VERSION" 1).

Detect if a plugin is installed

 $ nbdkit foo --version
 nbdkit: error: cannot open plugin 'foo': /usr/lib64/nbdkit/plugins/nbdkit-foo-plugin.so: cannot open shared object file: No such file or directory
 Use 'nbdkit --help' or read the nbdkit(1) manual page for documentation.

This will fail with an error and non-zero exit code if the "foo" plugin cannot be loaded.

Note it is better to test for the existence of plugins this way rather than just seeing if the .so file exists, because nbdkit will load the plugin and check that all its dependencies can be satisfied, and also that plugin registration works.

List all plugins in the plugin directory

However a better test is to run --dump-plugin (see above) on each one to check that it is working and all of its dependencies are installed. A complete shell script which does this is:

 #!/bin/sh -
 plugindir=`nbdkit --dump-config | grep ^plugindir= | sed 's/[^=]*=//'`
 for f in $plugindir/nbdkit-*-plugin.so; do
     if nbdkit "$f" --version >/dev/null 2>&1; then
         b=`echo "$f" | sed 's,.*/nbdkit-\(.*\)-plugin.so$,\1,'`
         echo "$b ($f)"
     fi
 done

Detect if a filter is installed

 nbdkit --version --filter=foo null

This will fail with an error and non-zero exit code if the "foo" filter cannot be loaded.