Sort kakoune selections using GNU sort utility

sort-selections.kak

define-command sort-selections -params 0.. -override -docstring '
sort-selections: Sort current selections using GNU sort utility
All parameters will be passed to the GNU sort utility
' %{
    # Copy current selections to a temporary sort buffer
    execute-keys %{"sy}
    edit -scratch *sort-selections*
    execute-keys %{"s<a-p>}

    # Seperate selections with null characters
    execute-keys %{<a-!>printf '\000'<ret>ggd}

    # Sort the buffer
    # Use `head -c-1` to get rid of leading newline
    execute-keys "%%|head -c-1|sort -z %arg{@}<ret>"

    # Yank the sorted candidates
    execute-keys %{%HS\x00<ret>"sy}

    # Update the original buffer
    delete-buffer
    execute-keys %{"sR}
}

How about this generalized version?

define-command pipe-selections -params 1.. -override -docstring '
sort-selections: Pipe all selections newline delimited through a shell command and return the result in place.
' %{
    # Copy current selections to a temporary sort buffer
    execute-keys %{"sy}
    edit -scratch *pipe-selections*
    execute-keys %{"s<a-P>a<ret><esc>gjd}

    # Sort the buffer
    execute-keys "%%|%arg{@}<ret>"

    # Yank the sorted candidates
    execute-keys %{%<a-s>H"sy}

    # Update the original buffer
    delete-buffer
    execute-keys %{"sR}
}