nbdkit-eval-plugin - write a shell script plugin on the command line
nbdkit eval get_size='SCRIPT' pread='SCRIPT' pwrite='SCRIPT' [...]
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).
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
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
'...' (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 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".
This is defined to the name of a temporary directory which can be used by the script snippets. It is deleted when nbdkit exits.
Shell script fragments are executed using /bin/sh.
nbdkit --dump-config to find the location of
nbdkit-eval-plugin first appeared in nbdkit 1.18.
nbdkit(1), nbdkit-plugin(3), nbdkit-sh-plugin(1). nbdkit-cc-plugin(1).
Richard W.M. Jones
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