Positional params passed to `preview_cmd`

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The README states that each line from list_cmd should provide at least three fields plus an optional fourth. preview_cmd then can access the first field as {1} and the fourth, optional, as {2}:

The list_cmd generated the list of entries. For each entry, it has to print the following columns, separated by the \034 field separator character:

The item to launch. This will get passed to preview_cmd and launch_cmd as {1}
The name of your provider (the same as what what you put inside the brackets, so my-provider in this example)
The text that appears in the fzf window. You might want to prepend it with a glyph and add some color via ANSI escape codes
(optional) Metadata that you can pass to preview_cmd and launch_cmd as {2}. For example, this is used to specify a specific Desktop Action inside a .desktop file
The preview_cmd renders the contents of the fzf preview panel. You can use the template variable {1} in your command, which will be substituted with the value of the selected item.

However, with a minimal test provider like this:

[test]
list_cmd=echo -e "item-to-launch1\034test\034fzf-text1\034optional-metadata1\nitem-to-launch2\034test\034fzf-text2\034optional-metadata2\n"
preview_cmd=echo "${@}"
launch_cmd=echo "na"
purge_cmd=echo "na"

The name of the provider (here, "test") is passed to preview_cmd as {1} and the item to launch is passed as {2}. Contrary to the docs, the optional metadata is not available to preview_cmd.

A simple, backwards-compatible, way of addressing this would be to pass the whole line from list_cmd through to preview_cmd from fzf ie. on l317

  --preview "$0 describe {2} {1} {+}"

(Plus updating the docs.)