Before: runtime autoload/ale/lsp.vim let g:args = [] " Mock the StartProgram function so we can just capture the arguments. function! ale#lsp#StartProgram(...) abort let g:args = a:000 endfunction After: unlet! g:args runtime autoload/ale/lsp.vim Execute(Command formatting should be applied correctly for LSP linters): call ale#lsp_linter#StartLSP( \ bufnr(''), \ { \ 'language_callback': {-> 'x'}, \ 'project_root_callback': {-> '/foo/bar'}, \ 'lsp': 'stdio', \ 'executable': has('win32') ? 'cmd': 'true', \ 'command': '%e --foo', \ }, \ {->0} \) if has('win32') AssertEqual \ ['cmd', 'cmd /s/c "cmd --foo"', '/foo/bar'], \ g:args[:2] else AssertEqual \ ['true', [&shell, '-c', '''true'' --foo'], '/foo/bar'], \ g:args[:2] endif