How to modify existing highlight group in vim?

If I have an existing highlight group in vim via link, for example

hi link my_highlight_group my_default_color

Is it possible to add 'bold' to my_highlight_group without changing my_default_color? Following does not work:

hi my_highlight_group gui=bold

Surprisingly, I can add bold if my_highlight group is defined directly (not via link):

hi my_highlight_group guifg=#F0000
hi my_highlight_group gui=bold

Solution

As "too much php" has said, there is no direct way to say "create a value that looks like that one and add bold". The best way is to modify your colour scheme. If you're not using a custom colour scheme, copy one from the main vim installation directory to your ~/.vim/colors directory and edit it to suit you. Alternatively, search on the vim scripts page and try some of the many that are available.

Shameless plug: if you want one that's easier to edit than the standard format, try my "Bandit" colour scheme.

If you really want to be able to add bold on the fly, you'll need a fairly complex script, like the one below. Note that this won't be saved for your next session unless you call it automatically after loading your colour scheme or by doing something like:

:autocmd ColorScheme AddBoldToGroup my_highlight_group

The script in all it's enormity is below. As far as I am aware, there is no significantly quicker way of doing this! Obviously you could save a few lines by writing less verbose code, but the general idea of using redir and silent hi repeatedly is the only way.

" Call this with something like
"
" :AddBoldToGroup perlRepeat
"
command! -complete=highlight -nargs=1 AddBoldToGroup call AddBoldToGroup(<f-args>)
function! AddBoldToGroup(group)
    " Redirect the output of the "hi" command into a variable
    " and find the highlighting
    redir => GroupDetails
    exe "silent hi " . a:group
    redir END

    " Resolve linked groups to find the root highlighting scheme
    while GroupDetails =~ "links to"
        let index = stridx(GroupDetails, "links to") + len("links to")
        let LinkedGroup =  strpart(GroupDetails, index + 1)
        redir => GroupDetails
        exe "silent hi " . LinkedGroup
        redir END
    endwhile

    " Extract the highlighting details (the bit after "xxx")
    let MatchGroups = matchlist(GroupDetails, '\<xxx\>\s\+\(.*\)')
    let ExistingHighlight = MatchGroups[1]

    " Check whether there's an existing gui= block
    let MatchGroups = matchlist(ExistingHighlight, '^\(.\{-}\) gui=\([^ ]\+\)\( .\{-}\)\?$')
    if MatchGroups != []
        " If there is, check whether "bold" is already in it
        let StartHighlight = MatchGroups[1]
        let GuiHighlight = MatchGroups[2]
        let EndHighlight = MatchGroups[3]
        if GuiHighlight =~ '.*bold.*'
            " Already done
            return
        endif
        " Add "bold" to the gui block
        let GuiHighlight .= ',bold'
        let NewHighlight = StartHighlight . GuiHighlight . EndHighlight
    else
        " If there's no GUI block, just add one with bold in it
        let NewHighlight = ExistingHighlight . " gui=bold"
    endif

    " Create the highlighting group
    exe "hi " . a:group . " " NewHighlight
endfunction