Manpages

NAME

nbdkit-probing - how to probe for nbdkit configuration and plugins

SYNOPSIS

nbdkit --dump-config

nbdkit PLUGIN --dump-plugin

nbdkit --version

nbdkit PLUGIN --version

nbdkit --filter=FILTER null --version

nbdkit --filter=FILTER null --dump-plugin

DESCRIPTION

You can query information about nbdkit and available plugins and filters using the nbdkit binary. This can include whether nbdkit is installed, and whether plugins or filters are installed.

Query if nbdkit is installed
Use this command to see if the nbdkit program 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
To test if nbdkit ≥ a particular version is installed, use the --dump-config option and look for the "version_major" and "version_minor" fields:

$ 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.

You can also probe the minimum version using pkg-config(1). See "PKG-CONFIG/PKGCONF" in nbdkit-plugin(3).

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).

Query information about a particular filter
You can use the --dump-plugin option to dump information about a filter, by using the null plugin with the --filter parameter:

$ nbdkit null --filter=qcow2dec --dump-plugin
[...]
qcow2dec_path=/usr/lib64/nbdkit/filters/nbdkit-qcow2dec-filter.so
qcow2dec_name=qcow2dec
qcow2dec_deflate=yes
qcow2dec_zstd=yes

Detect if a plugin is installed
To find out if a plugin is installed (and working) in the plugin directory, use:

$ 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
You could simply get the plugin directory (from --dump-config) and list all files in this directory called nbdkit-*-plugin.so.

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
To find out if a filter is installed (and working) use --version with the "null" plugin and the name of the filter to test:

nbdkit --version --filter=foo null

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

SEE ALSO

nbdkit(1).

AUTHORS

Eric Blake

Richard W.M. Jones

Pino Toscano

COPYRIGHT

Copyright Red Hat

LICENSE

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