1nbdkit-eval-plugin(1) NBDKIT nbdkit-eval-plugin(1)
2
6 nbdkit-eval-plugin - write a shell script plugin on the command line
7
9 nbdkit eval get_size='SCRIPT' pread='SCRIPT' pwrite='SCRIPT' [...]
10
12 "nbdkit-eval-plugin" is an nbdkit(1) plugin which allows you to write
13 custom plugins as shell scripts snippets ‘eval’d on the command line.
14 A common alternative to this plugin is nbdkit-sh-plugin(1). Both
16 plugins share the same source code and work in almost the same way.
17 You should read nbdkit-sh-plugin(1) first. It is easier to describe
18 the differences between the two plugins and look at the examples below.
19 • nbdkit-sh-plugin(1) plugins are written as a single script in a
21 separate file. Eval plugins are shell script fragments written on
22 the nbdkit command line — there is no separate script file.
23 • nbdkit-sh-plugin(1) has no way to know if a method is missing or
25 not and so each "can_*" method (eg. "can_write") must be written
26 explicitly. In eval plugins you have the option of omitting
27 "can_*" methods if the associated callback (eg. "pwrite") is
28 defined. In this way eval plugins work more like regular nbdkit
29 plugins.
30 • Eval plugins can only use /bin/sh to run the script snippets, but
32 nbdkit-sh-plugin(1) (in spite of the name) can run any executable.
33 • There is no "load" method (although there is an "unload" method and
35 all other methods are identical).
36
38 Create a 64M read-only disk of zeroes:
39 nbdkit eval get_size=' echo 64M ' \
41 pread=' dd if=/dev/zero count=$3 iflag=count_bytes '
42 The following command is the eval plugin equivalent of
44 nbdkit-file-plugin(1) (except not as fast and missing many features):
45 nbdkit eval \
47 config='ln -sf "$(realpath "$3")" $tmpdir/file' \
48 get_size='stat -Lc %s $tmpdir/file' \
49 pread='dd if=$tmpdir/file skip=$4 count=$3 iflag=count_bytes,skip_bytes' \
50 pwrite='dd of=$tmpdir/file seek=$4 conv=notrunc oflag=seek_bytes' \
51 file=disk.img
52
54 after_fork=SCRIPT
55 cache=SCRIPT
56 can_cache=SCRIPT
57 can_extents=SCRIPT
58 can_fast_zero=SCRIPT
59 can_flush=SCRIPT
60 can_fua=SCRIPT
61 can_multi_conn=SCRIPT
62 can_trim=SCRIPT
63 can_write=SCRIPT
64 can_zero=SCRIPT
65 close=SCRIPT
66 config=SCRIPT
67 config_complete=SCRIPT
68 default_export=SCRIPT
69 dump_plugin=SCRIPT
70 export_description=SCRIPT
71 extents=SCRIPT
72 flush=SCRIPT
73 get_ready=SCRIPT
74 get_size=SCRIPT
75 is_rotational=SCRIPT
76 list_exports=SCRIPT
77 default_export=SCRIPT
78 open=SCRIPT
79 pread=SCRIPT
80 preconnect=SCRIPT
81 pwrite=SCRIPT
82 thread_model=SCRIPT
83 trim=SCRIPT
84 unload=SCRIPT
85 zero=SCRIPT
86 Define the script associated with each method. "SCRIPT" is a
87 fragment of shell script which is executed when nbdkit wants to
88 invoke the associated method.
89 If you are typing these commands at the shell, be careful about
91 quoting. Normally you will need to enclose "SCRIPT" in '...'
92 (single quotes) to prevent it from being modified by your shell.
93 The script fragment behaves the same way as the corresponding
95 method in nbdkit-sh-plugin(1). In particular, parameters are
96 identical, $tmpdir is present and used in the same way, the exit
97 code must be one of the valid exit codes described in that manual
98 page, and error handling works the same way too.
99 Note that a "config" callback will only handle keys not recognized
101 as callback names; when picking key=value pairs that you want your
102 script fragment to understand, be aware that if a future nbdkit
103 release creates a callback by that name, your "config" script
104 fragment will no longer see that key.
105 All of these parameters are optional.
107 missing=SCRIPT
109 The parameter "missing" defines a script that will be called in
110 place of any other callback not explicitly provided. If omitted,
111 this defaults to the script "exit 2".
112
114 "tmpdir"
115 This is defined to the name of a temporary directory which can be
116 used by the script snippets. It is deleted when nbdkit exits.
117
119 /bin/sh
120 Shell script fragments are executed using /bin/sh.
121 $plugindir/nbdkit-eval-plugin.so
123 The plugin.
124 Use "nbdkit --dump-config" to find the location of $plugindir.
126
128 "nbdkit-eval-plugin" first appeared in nbdkit 1.18.
129
131 nbdkit(1), nbdkit-plugin(3), nbdkit-sh-plugin(1). nbdkit-cc-plugin(1).
132
134 Richard W.M. Jones
135
137 Copyright (C) 2019 Red Hat Inc.
138
140 Redistribution and use in source and binary forms, with or without
141 modification, are permitted provided that the following conditions are
142 met:
143 • Redistributions of source code must retain the above copyright
145 notice, this list of conditions and the following disclaimer.
146 • Redistributions in binary form must reproduce the above copyright
148 notice, this list of conditions and the following disclaimer in the
149 documentation and/or other materials provided with the
150 distribution.
151 • Neither the name of Red Hat nor the names of its contributors may
153 be used to endorse or promote products derived from this software
154 without specific prior written permission.
155 THIS SOFTWARE IS PROVIDED BY RED HAT AND CONTRIBUTORS ''AS IS'' AND ANY
157 EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
158 IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
159 PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL RED HAT OR CONTRIBUTORS BE
160 LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
161 CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
162 SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR
163 BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
164 WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR
165 OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
166 ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
167nbdkit-1.28.2 2021-11-09 nbdkit-eval-plugin(1)