Ubuntu Manpage: nbdkit-captive - run nbdkit under another process and have it reliably cleaned up

Provided by: nbdkit_1.42.6-1ubuntu1_amd64

NAME

       nbdkit-captive - run nbdkit under another process and have it reliably cleaned up

SYNOPSIS

        nbdkit PLUGIN [...] [-e|--exportname EXPORTNAME] \
                            --run 'COMMAND ARGS ...'

        nbdkit --exit-with-parent PLUGIN [...]

DESCRIPTION

       You can run nbdkit under another process and have nbdkit reliably clean up.  There are two techniques
       depending on whether you want nbdkit to start the other process ("CAPTIVE NBDKIT"), or if you want the
       other process to start nbdkit ("EXIT WITH PARENT").  Another way is to have nbdkit exit after the last
       client connection (nbdkit-exitlast-filter(1)) or after an event (nbdkit-exitwhen-filter(1)).

CAPTIVE NBDKIT

You can run nbdkit as a "captive process", using the --run option. This means that nbdkit runs as long
as (for example) qemu(1) or guestfish(1) is running. When those exit, nbdkit is killed.

Some examples should make this clear.

To run nbdkit captive under qemu:

nbdkit file disk.img --run 'qemu -drive file="$uri",if=virtio'

On the qemu command line, $uri is substituted automatically with the right NBD path so it can connect to
nbdkit. When qemu exits, nbdkit is killed and cleaned up automatically.

Running nbdkit captive under guestfish:

nbdkit file disk.img --run 'guestfish --format=raw -a "$uri" -i'

When guestfish exits, nbdkit is killed.

Running nbdkit captive under nbdsh for unit testing:

nbdkit memory 1 --run 'nbdsh -u "$uri" -c "print(h.pread(1, 0))"'

Running an fio benchmark:

nbdkit -f null 1G --run 'export uri; fio fio.git/examples/nbd.fio'

The following shell variables are available in the --run argument (local to the run script unless you use
"export"):

$uri
$nbd
A URI that refers to the nbdkit port or socket in the preferred form documented by the NBD project.

As this variable may contain a bare "?" for Unix sockets, it is safest to use $uri within double
quotes to avoid unintentional globbing. For plugins that support distinct data based on export
names, the -e option to nbdkit controls which export name will be set in the URI.

In nbdkit ≤ 1.22 $nbd tried to guess if you were using qemu or guestfish and expanded differently.
Since NBD URIs are now widely supported this magic is no longer necessary. In nbdkit ≥ 1.24 both
variables expand to the same URI.

See also "NBD URIs and endpoints" in nbdkit(1).

$port
If ≠ "", the port number that nbdkit is listening on.

$unixsocket
If ≠ "", the Unix domain socket that nbdkit is listening on.

$exportname
The export name (which may be "") that the process should use when connecting to nbdkit, as set by
the -e (--exportname) command line option of nbdkit. This only matters to plugins that differentiate
what they serve based on the export name requested by the client.

$tls
Corresponds to the --tls option passed to nbdkit. If --tls=off this is not set. If --tls=on this is
set to "1". If --tls=require this is set to "2".

$tls_certificates
If --tls-certificates was passed to nbdkit, the value is copied here. It is usually the directory
containing PKI certificates. Note that the path might not be an absolute path, or even valid.

$tls_psk
If --tls-psk was passed to nbdkit, the value is copied here. It is usually the filename of a TLS
Pre-Shared Keys (PSK) file. Note that the filename might not be an absolute path, or even valid.

--run implies --foreground. It is not possible, and probably not desirable, to have nbdkit fork into the
background when using --run.

Copying data in and out of plugins with captive nbdkit
Captive nbdkit + qemu-img(1) can be used to copy data into and out of nbdkit plugins. For example
nbdkit-example1-plugin(1) contains an embedded disk image. To copy it out:

nbdkit example1 --run 'qemu-img convert "$uri" disk.img'

If the source suffers from temporary network failures nbdkit-retry-filter(1) or
nbdkit-retry-request-filter(1) may help.

To overwrite a file inside an uncompressed tar file (the file being overwritten must be the same size),
use nbdkit-tar-filter(1) like this:

nbdkit file data.tar --filter=tar tar-entry=disk.img \
--run 'qemu-img convert -n disk.img "$uri"'

EXIT WITH PARENT

The --exit-with-parent option is almost the opposite of "CAPTIVE NBDKIT" described in the previous
section.

Running nbdkit with this option, for example from a script:

nbdkit --exit-with-parent plugin ... &

means that nbdkit will exit automatically if the parent program exits for any reason. This can be used
to avoid complicated cleanups or orphaned nbdkit processes.

--exit-with-parent is incompatible with forking into the background (because when we fork into the
background we lose track of the parent process). Therefore -f / --foreground is implied.

If the parent application is multithreaded, then (in the Linux implementation) if the parent thread
exits, that will cause nbdkit to exit. Thus in multithreaded applications you usually want to run
"nbdkit --exit-with-parent" only from the main thread (unless you actually want nbdkit to exit with the
thread, but that may not work reliably on all operating systems).

To exit when an unrelated process exits, use nbdkit-exitwhen-filter(1) "exit-when-process-exits" feature.

Support for --exit-with-parent
This is currently implemented using a non-POSIX feature available in Linux ≥ 2.1.57, FreeBSD ≥ 11.2 and
macOS. It won't work on other operating systems (patches welcome to make it work).

To test if the current binary supports this feature the most backwards compatible way is:

nbdkit --exit-with-parent --version && echo "supported"

In nbdkit ≥ 1.34, "nbdkit --dump-config" prints either "exit_with_parent=yes" or "exit_with_parent=no"
but earlier versions did not have this.

AUTHORS

       Eric Blake

       Richard W.M. Jones

       Pino Toscano

COPYRIGHT

       Copyright Red Hat

LICENSE

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       •   Neither the name of Red Hat nor the names of its contributors may  be  used  to  endorse  or  promote
           products derived from this software without specific prior written permission.

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nbdkit-1.42.6                                      2025-08-01                                  nbdkit-captive(1)