1nbdkit-captive(1) NBDKIT nbdkit-captive(1)
2
6 nbdkit-captive - run nbdkit under another process and have it reliably
7 cleaned up
8
10 nbdkit PLUGIN [...] [-e|--exportname EXPORTNAME] --run "CMD ARGS ..."
11 nbdkit --exit-with-parent PLUGIN [...]
13
15 You can run nbdkit under another process and have nbdkit reliably clean
16 up. There are two techniques depending on whether you want nbdkit to
17 start the other process ("CAPTIVE NBDKIT"), or if you want the other
18 process to start nbdkit ("EXIT WITH PARENT"). Another way is to have
19 nbdkit exit after the last client connection
20 (nbdkit-exitlast-filter(1)) or after an event
21 (nbdkit-exitwhen-filter(1)).
22
24 You can run nbdkit as a "captive process", using the --run option.
25 This means that nbdkit runs as long as (for example) qemu(1) or
26 guestfish(1) is running. When those exit, nbdkit is killed.
27 Some examples should make this clear.
29 To run nbdkit captive under qemu:
31 nbdkit file disk.img --run 'qemu -drive file=$nbd,if=virtio'
33 On the qemu command line, $nbd is substituted automatically with the
35 right NBD path so it can connect to nbdkit. When qemu exits, nbdkit is
36 killed and cleaned up automatically.
37 Running nbdkit captive under guestfish:
39 nbdkit file disk.img --run 'guestfish --format=raw -a $nbd -i'
41 When guestfish exits, nbdkit is killed.
43 Running nbdkit captive under nbdsh for unit testing:
45 nbdkit -U - memory 1 --run 'nbdsh -u "$uri" -c "print(h.pread(1, 0))"'
47 The following shell variables are available in the --run argument:
49 $nbd
51 $uri
52 A URI that refers to the nbdkit port or socket in the preferred
53 form documented by the NBD project.
54 As this variable may contain a bare "?" for Unix sockets, it is
56 safest to use $uri within double quotes to avoid unintentional
57 globbing. For plugins that support distinct data based on export
58 names, the -e option to nbdkit controls which export name will be
59 set in the URI.
60 In nbdkit ≤ 1.22 $nbd tried to guess if you were using qemu or
62 guestfish and expanded differently. Since NBD URIs are now widely
63 supported this magic is no longer necessary. In nbdkit ≥ 1.24 both
64 variables expand to the same URI.
65 $tls
67 Corresponds to the --tls option passed to nbdkit. If --tls=off
68 this is not set. If --tls=on this is set to "1". If --tls=require
69 this is set to "2".
70 $port
72 If ≠ "", the port number that nbdkit is listening on.
73 $unixsocket
75 If ≠ "", the Unix domain socket that nbdkit is listening on.
76 $exportname
78 The export name (which may be "") that the process should use when
79 connecting to nbdkit, as set by the -e (--exportname) command line
80 option of nbdkit. This only matters to plugins that differentiate
81 what they serve based on the export name requested by the client.
82 --run implies --foreground. It is not possible, and probably not
84 desirable, to have nbdkit fork into the background when using --run.
85 Even when running captive, nbdkit still listens on the regular TCP/IP
87 port, unless you specify the -p/-U options. If you want a truly
88 private captive nbdkit, then you should create a private random Unix
89 socket, like this:
90 nbdkit -U - plugin [args] --run '...'
92 Copying data in and out of plugins with captive nbdkit
94 Captive nbdkit + qemu-img(1) can be used to copy data into and out of
95 nbdkit plugins. For example nbdkit-example1-plugin(1) contains an
96 embedded disk image. To copy it out:
97 nbdkit -U - example1 --run 'qemu-img convert $nbd disk.img'
99 If plugin requests have a high overhead (for example making HTTP
101 requests to a remote server), adding nbdkit-readahead-filter(1) may
102 help performance:
103 nbdkit -U - --filter=readahead curl https://example.com/disk.img \
105 --run 'qemu-img convert $nbd disk.img'
106 If the source suffers from temporary network failures
108 nbdkit-retry-filter(1) or nbdkit-retry-request-filter(1) may help.
109 To overwrite a file inside an uncompressed tar file (the file being
111 overwritten must be the same size), use nbdkit-tar-filter(1) like this:
112 nbdkit -U - file data.tar --filter=tar tar-entry=disk.img \
114 --run 'qemu-img convert -n disk.img $nbd'
115
117 The --exit-with-parent option is almost the opposite of "CAPTIVE
118 NBDKIT" described in the previous section.
119 Running nbdkit with this option, for example from a script:
121 nbdkit --exit-with-parent plugin ... &
123 means that nbdkit will exit automatically if the parent program exits
125 for any reason. This can be used to avoid complicated cleanups or
126 orphaned nbdkit processes.
127 --exit-with-parent is incompatible with forking into the background
129 (because when we fork into the background we lose track of the parent
130 process). Therefore -f / --foreground is implied.
131 This is currently implemented using a non-POSIX feature available in
133 Linux ≥ 2.1.57 and FreeBSD ≥ 11.2, so it won't work on other operating
134 systems (patches welcome to make it work).
135 If the parent application is multithreaded, then (in the Linux
137 implementation) if the parent thread exits, that will cause nbdkit to
138 exit. Thus in multithreaded applications you usually want to run
139 "nbdkit --exit-with-parent" only from the main thread (unless you
140 actually want nbdkit to exit with the thread, but that may not work
141 reliably on all operating systems).
142 To exit when an unrelated process exits, use nbdkit-exitwhen-filter(1)
144 "exit-when-process-exits" feature.
145
147 nbdkit(1), nbdkit-exitlast-filter(1), nbdkit-exitwhen-filter(1),
148 prctl(2) (on Linux), procctl(2) (on FreeBSD).
149
151 Eric Blake
152 Richard W.M. Jones
154 Pino Toscano
156
158 Copyright (C) 2013-2020 Red Hat Inc.
159
161 Redistribution and use in source and binary forms, with or without
162 modification, are permitted provided that the following conditions are
163 met:
164 • Redistributions of source code must retain the above copyright
166 notice, this list of conditions and the following disclaimer.
167 • Redistributions in binary form must reproduce the above copyright
169 notice, this list of conditions and the following disclaimer in the
170 documentation and/or other materials provided with the
171 distribution.
172 • Neither the name of Red Hat nor the names of its contributors may
174 be used to endorse or promote products derived from this software
175 without specific prior written permission.
176 THIS SOFTWARE IS PROVIDED BY RED HAT AND CONTRIBUTORS ''AS IS'' AND ANY
178 EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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180 PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL RED HAT OR CONTRIBUTORS BE
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188nbdkit-1.30.7 2022-07-10 nbdkit-captive(1)