change ale to use value of g:ale_shell regardless of what it is

autoload/ale/job.vim

@@ -12,10 +12,10 @@
 let g:ale_command_wrapper = get(g:, 'ale_command_wrapper', '')
 
 " A setting for the shell used to execute commands
-let g:ale_shell = get(g:, 'ale_shell', &shell)
+let g:ale_shell = get(g:, 'ale_shell', v:null)
 
 " A setting for the arguments we pass to the shell when executing commands
-let g:ale_shell_arguments = get(g:, 'ale_shell_arguments', &shellcmdflag)
+let g:ale_shell_arguments = get(g:, 'ale_shell_arguments', v:null)
 
 if !has_key(s:, 'job_map')
     let s:job_map = {}
@@ -190,15 +190,27 @@ function! ale#job#PrepareCommand(buffer, command) abort
     " NeoVim handles this issue automatically if the command is a String,
     " but we'll do this explicitly, so we use the same exact command for both
     " versions.
-    if has('win32')
-        return 'cmd /s/c "' . l:command . '"'
-    endif
+    if g:ale_shell is v:null
+      if has('win32')
+          return 'cmd /s/c "' . l:command . '"'
+      endif
 
-    if g:ale_shell =~? 'fish$\|pwsh$'
-        return ['/bin/sh', '-c', l:command]
-    endif
+      if &shell =~? 'fish$\|pwsh$'
+          return ['/bin/sh', '-c', l:command]
+      endif
 
-    return [g:ale_shell] + split(g:ale_shell_arguments) + [l:command]
+      return split(&shell) + split(&shellcmdflag) + [l:command]
+    else
+      if has('win32')
+          return g:ale_shell . l:command . '"'
+      endif
+
+      let l:shell_arguments = g:ale_shell_arguments is v:null
+      \ ? &shellcmdflag
+      \ : g:ale_shell_arguments
+
+      return split(g:ale_shell) + split(l:shell_arguments) + [l:command]
+    endif
 endfunction
 
 " Start a job with options which are agnostic to Vim and NeoVim.

doc/ale.txt

@@ -1993,14 +1993,17 @@ g:ale_windows_node_executable_path         *g:ale_windows_node_executable_path*
 g:ale_shell                                                       *g:ale_shell*
 
   Type: |String|
-  Default: `'&shell'`
+  Default: not set
 
   This variable is used to determine which shell ale will use to execute 
-  commands. This variables defaults to the value of the vim option '&shell'
-  which corresponds to the $SHELL environment variable. For example 
-  if `$SHELL == '/bin/bash'`, but you want to use zsh, set `g:ale_shell = '/bin/zsh'.`
-  If this option is set to either `fish` or `pwsh` (either by default or set
-  with this command), ale will still execute commands with `bin/sh`. 
+  commands. By default this variable is undefined, meaning that ALE will use
+  it's default behavior. Which is to run shells via the shell determined by
+  the `&shell` vim variable, with the arguments `&shellcmdflag`. Ale will fall
+  back to using `/bin/sh`if it detects the underlying `&shell`is either `fish`
+  or `pwsh`. However, if you set this variable ALE will no longer fall back to
+  other shells, meaning if you wanted to use `fish` you could do so via this
+  option. For example if `$SHELL == '/bin/bash'`, but you want to use zsh, 
+  set `g:ale_shell = '/bin/zsh'. 
 
   Please note - if you are using this option you should consider additionally 
   setting `g:ale``g:ale_shell_arguments` since the default values for that
@@ -2009,12 +2012,11 @@ g:ale_shell                                                       *g:ale_shell*
 g:ale_shell_arguments                                   *g:ale_shell_arguments*
 
   Type: |String|
-  Default: `'&shellcmdflag'`
+  Default: not set
 
   This variable is used to determine what commands vim will pass to the shell
-  to execute it's commands. By default, `&shellcmdflag` would be set to the
-  value of '`-c'`. Similar to `g:ale_shell`, if the shell is set to either
-  `fish` or `pwsh`, `g:ale_shell_arguments` will no be used.
+  to execute it's commands. If this command is not set, but g:ale_shell is
+  set, ale will use `&shellcmdflag` as command arguments.
 
 -------------------------------------------------------------------------------
 6.1. Highlights                                                *ale-highlights*

test/fix/test_ale_fix.vader

@@ -26,7 +26,7 @@ Before:
   augroup END
 
   if !has('win32')
-    let g:ale_shell = '/bin/bash'
+    let &shell = '/bin/bash'
   endif
 
   call ale#test#SetDirectory('/testplugin/test')

test/lsp/test_lsp_command_formatting.vader

@@ -31,6 +31,6 @@ Execute(Command formatting should be applied correctly for LSP linters):
     \ g:args
   else
     AssertEqual
-    \ ['true', [g:ale_shell, '-c', '''true'' --foo']],
+    \ ['true', [&shell, '-c', '''true'' --foo']],
     \ g:args
   endif

test/test_prepare_command.vader

@@ -4,7 +4,7 @@ Before:
 
 After:
   Restore
-  let g:ale_shell = &shell
+  let g:ale_shell = v:null
 
 Execute(sh should be used when the shell is fish):
   if !has('win32')