Update: shellcheck now has a gcc-style output mode.

syntax_checkers/sh/shellcheck.vim

@@ -8,29 +8,21 @@ if exists("g:loaded_syntastic_sh_shellcheck_checker")
 endif
 let g:loaded_syntastic_sh_shellcheck_checker = 1
 
-function! SyntaxCheckers_sh_shellcheck_Preprocess(json)
-    " A hat tip to Mark Weber for this trick
-    " http://stackoverflow.com/a/19105763
-    let errors = eval(join(a:json, ''))
-
-    call filter(errors, 'v:val["level"] =~? ''\v^(error|warning|style)$''')
-    return map(errors, 'v:val["level"][0] . ":" . v:val["line"] . ":" . v:val["column"] . ":" . v:val["message"]')
-endfunction
-
 function! SyntaxCheckers_sh_shellcheck_GetLocList() dict
-    let makeprg = self.makeprgBuild({ 'args': '-f json' })
+    let makeprg = self.makeprgBuild({ 'args': '-f gcc' })
 
-    let errorformat = '%t:%l:%v:%m'
+    let errorformat =
+        \ '%f:%l:%v: %trror: %m,' .
+        \ '%f:%l:%v: %tarning: %m,' .
+        \ '%f:%l:%v: %tote: %m'
 
     let loclist = SyntasticMake({
         \ 'makeprg': makeprg,
         \ 'errorformat': errorformat,
-        \ 'preprocess': 'SyntaxCheckers_sh_shellcheck_Preprocess',
-        \ 'defaults': {'bufnr': bufnr("")},
         \ 'returns': [0, 1] })
 
     for e in loclist
-        if e['type'] ==? 's'
+        if e['type'] ==? 'n'
             let e['type'] = 'w'
             let e['subtype'] = 'Style'
         endif