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

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       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.

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