Provided by: nbdkit_1.36.3-1ubuntu10_amd64 
NAME
nbdkit-eval-plugin - write a shell script plugin on the command line
SYNOPSIS
nbdkit eval get_size='SCRIPT' pread='SCRIPT' pwrite='SCRIPT' [...]
DESCRIPTION
"nbdkit-eval-plugin" is an nbdkit(1) plugin which allows you to write custom plugins as shell scripts
snippets ‘eval’d on the command line.
A common alternative to this plugin is nbdkit-sh-plugin(1). Both plugins share the same source code and
work in almost the same way. You should read nbdkit-sh-plugin(1) first. It is easier to describe the
differences between the two plugins and look at the examples below.
• nbdkit-sh-plugin(1) plugins are written as a single script in a separate file. Eval plugins are
shell script fragments written on the nbdkit command line — there is no separate script file.
• nbdkit-sh-plugin(1) has no way to know if a method is missing or not and so each "can_*" method (eg.
"can_write") must be written explicitly. In eval plugins you have the option of omitting "can_*"
methods if the associated callback (eg. "pwrite") is defined. In this way eval plugins work more
like regular nbdkit plugins.
• Eval plugins can only use /bin/sh to run the script snippets, but nbdkit-sh-plugin(1) (in spite of
the name) can run any executable.
• There is no "load" method (although there is an "unload" method and all other methods are identical).
EXAMPLES
Create a 64M read-only disk of zeroes:
nbdkit eval get_size=' echo 64M ' \
pread=' dd if=/dev/zero count=$3 iflag=count_bytes '
The following command is the eval plugin equivalent of nbdkit-file-plugin(1) (except not as fast and
missing many features):
nbdkit eval \
config='ln -sf "$(realpath "$3")" $tmpdir/file' \
get_size='stat -Lc %s $tmpdir/file' \
pread='dd if=$tmpdir/file skip=$4 count=$3 iflag=count_bytes,skip_bytes' \
pwrite='dd of=$tmpdir/file seek=$4 conv=notrunc oflag=seek_bytes' \
file=disk.img
PARAMETERS
after_fork=SCRIPT
block_size=SCRIPT
cache=SCRIPT
can_cache=SCRIPT
can_extents=SCRIPT
can_fast_zero=SCRIPT
can_flush=SCRIPT
can_fua=SCRIPT
can_multi_conn=SCRIPT
can_trim=SCRIPT
can_write=SCRIPT
can_zero=SCRIPT
close=SCRIPT
config=SCRIPT
config_complete=SCRIPT
default_export=SCRIPT
dump_plugin=SCRIPT
export_description=SCRIPT
extents=SCRIPT
flush=SCRIPT
get_ready=SCRIPT
get_size=SCRIPT
is_rotational=SCRIPT
list_exports=SCRIPT
default_export=SCRIPT
open=SCRIPT
pread=SCRIPT
preconnect=SCRIPT
pwrite=SCRIPT
thread_model=SCRIPT
trim=SCRIPT
unload=SCRIPT
zero=SCRIPT
Define the script associated with each method. "SCRIPT" is a fragment of shell script which is
executed when nbdkit wants to invoke the associated method.
If you are typing these commands at the shell, be careful about quoting. Normally you will need to
enclose "SCRIPT" in '...' (single quotes) to prevent it from being modified by your shell.
The script fragment behaves the same way as the corresponding method in nbdkit-sh-plugin(1). In
particular, parameters are identical, $tmpdir is present and used in the same way, the exit code must
be one of the valid exit codes described in that manual page, and error handling works the same way
too. Likewise, nbdkit --dump-plugin eval includes a line for max_known_status= in nbdkit ≥ 1.34.
Note that a "config" callback will only handle keys not recognized as callback names; when picking
key=value pairs that you want your script fragment to understand, be aware that if a future nbdkit
release creates a callback by that name, your "config" script fragment will no longer see that key.
All of these parameters are optional.
missing=SCRIPT
The parameter "missing" defines a script that will be called in place of any other callback not
explicitly provided. If omitted, this defaults to the script "exit 2".
ENVIRONMENT VARIABLES
"tmpdir"
This is defined to the name of a temporary directory which can be used by the script snippets. It is
deleted when nbdkit exits.
FILES
/bin/sh
Shell script fragments are executed using /bin/sh.
$plugindir/nbdkit-eval-plugin.so
The plugin.
Use "nbdkit --dump-config" to find the location of $plugindir.
VERSION
"nbdkit-eval-plugin" first appeared in nbdkit 1.18.
SEE ALSO
nbdkit(1), nbdkit-plugin(3), nbdkit-sh-plugin(1). nbdkit-cc-plugin(1).
AUTHORS
Richard W.M. Jones
COPYRIGHT
Copyright Red Hat
LICENSE
Redistribution and use in source and binary forms, with or without modification, are permitted provided
that the following conditions are met:
• Redistributions of source code must retain the above copyright notice, this list of conditions and
the following disclaimer.
• Redistributions in binary form must reproduce the above copyright notice, this list of conditions and
the following disclaimer in the documentation and/or other materials provided with the distribution.
• 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.
THIS SOFTWARE IS PROVIDED BY RED HAT AND CONTRIBUTORS ''AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES,
INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL RED HAT OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR
OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH
DAMAGE.
nbdkit-1.36.3 2024-03-31 nbdkit-eval-plugin(1)