jammy (1) nbdkit-eval-plugin.1.gz

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
       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.

           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 (C) 2019 Red Hat Inc.

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.