Fix #1051 - Support ash and dash for shellcheck and the sh linter

ale_linters/sh/shellcheck.vim

@@ -23,15 +23,20 @@ function! ale_linters#sh#shellcheck#GetDialectArgument(buffer) abort
     let l:shell_type = ale#handlers#sh#GetShellType(a:buffer)
 
     if !empty(l:shell_type)
+        " Use the dash dialect for /bin/ash, etc.
+        if l:shell_type is# 'ash'
+            return 'dash'
+        endif
+
         return l:shell_type
     endif
 
     " If there's no hashbang, try using Vim's buffer variables.
-    if get(b:, 'is_bash')
+    if getbufvar(a:buffer, 'is_bash', 0)
         return 'bash'
-    elseif get(b:, 'is_sh')
+    elseif getbufvar(a:buffer, 'is_sh', 0)
         return 'sh'
-    elseif get(b:, 'is_kornshell')
+    elseif getbufvar(a:buffer, 'is_kornshell', 0)
         return 'ksh'
     endif
 

autoload/ale/handlers/sh.vim

@@ -9,7 +9,7 @@ function! ale#handlers#sh#GetShellType(buffer) abort
         " Remove options like -e, etc.
         let l:command = substitute(l:bang_line, ' --\?[a-zA-Z0-9]\+', '', 'g')
 
-        for l:possible_shell in ['bash', 'tcsh', 'csh', 'zsh', 'sh']
+        for l:possible_shell in ['bash', 'dash', 'ash', 'tcsh', 'csh', 'zsh', 'sh']
             if l:command =~# l:possible_shell . '\s*$'
                 return l:possible_shell
             endif

test/test_shell_detection.vader

@@ -81,3 +81,23 @@ Execute(The ksh dialect should be used for shellcheck if b:is_kornshell is 1):
   let b:is_kornshell = 1
 
   AssertEqual 'ksh', ale_linters#sh#shellcheck#GetDialectArgument(bufnr(''))
+
+Given(A file with /bin/ash):
+  #!/bin/ash
+
+Execute(The ash dialect should be used for the shell and the base function):
+  AssertEqual 'ash', ale#handlers#sh#GetShellType(bufnr(''))
+  AssertEqual 'ash', ale_linters#sh#shell#GetExecutable(bufnr(''))
+
+Execute(dash should be used for shellcheck, which has no ash dialect):
+  AssertEqual 'dash', ale_linters#sh#shellcheck#GetDialectArgument(bufnr(''))
+
+Given(A file with /bin/dash):
+  #!/bin/dash
+
+Execute(The dash dialect should be used for the shell and the base function):
+  AssertEqual 'dash', ale#handlers#sh#GetShellType(bufnr(''))
+  AssertEqual 'dash', ale_linters#sh#shell#GetExecutable(bufnr(''))
+
+Execute(dash should be used for shellcheck):
+  AssertEqual 'dash', ale_linters#sh#shellcheck#GetDialectArgument(bufnr(''))