Add support for ale option to override default shell used by ale

autoload/ale/job.vim

@@ -11,6 +11,12 @@
 " A setting for wrapping commands.
 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', 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', v:null)
+
 if !has_key(s:, 'job_map')
     let s:job_map = {}
 endif
@@ -184,15 +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 &shell =~? 'fish$\|pwsh$'
-        return ['/bin/sh', '-c', l:command]
-    endif
+      if &shell =~? 'fish$\|pwsh$'
+          return ['/bin/sh', '-c', l:command]
+      endif
 
-    return split(&shell) + split(&shellcmdflag) + [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

@@ -1990,6 +1990,33 @@ g:ale_windows_node_executable_path         *g:ale_windows_node_executable_path*
   scripts are executed with whatever executable is configured with this
   setting.
 
+g:ale_shell                                                       *g:ale_shell*
+
+  Type: |String|
+  Default: not set
+
+  This variable is used to determine which shell ale will use to execute 
+  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
+  option might be incompatable with the newly set shell. 
+
+g:ale_shell_arguments                                   *g:ale_shell_arguments*
+
+  Type: |String|
+  Default: not set
+
+  This variable is used to determine what commands vim will pass to the shell
+  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/test_prepare_command.vader

@@ -4,6 +4,7 @@ Before:
 
 After:
   Restore
+  let g:ale_shell = v:null
 
 Execute(sh should be used when the shell is fish):
   if !has('win32')
@@ -43,6 +44,7 @@ Execute(Other shells should be used when set):
   if !has('win32')
     let &shell = '/bin/bash'
     let &shellcmdflag = '-c'
+    let g:ale_shell = &shell
 
     AssertEqual ['/bin/bash', '-c', 'foobar'], ale#job#PrepareCommand(bufnr(''), 'foobar')
   endif
@@ -54,3 +56,10 @@ Execute(cmd /s/c as a string should be used on Windows):
 
     AssertEqual 'cmd /s/c "foobar"', ale#job#PrepareCommand(bufnr(''), 'foobar')
   endif
+
+Execute(Setting ale_shell should cause ale#job#PrepareCommand to use set shell):
+  if !has('win32')
+    let g:ale_shell = '/foo/bar'
+
+    AssertEqual ['/foo/bar', '-c', 'foobar'], ale#job#PrepareCommand(bufnr(''), "foobar")
+  endif