runtime(sh): consider sh as POSIX shell by default

Commit: 
https://github.com/vim/vim/commit/51a06ecee06096672b2f10fc6cd76bd8f6dfe8c9
Author: Mohamed Akram <mohd.ak...@outlook.com>
Date:   Fri Mar 21 17:52:08 2025 +0100

    runtime(sh): consider sh as POSIX shell by default
    
    Also, do not set g:is_kornshell when g:is_posix is set. BSD shells are
    POSIX but many are derived from the ash shell.
    
    closes: #16939
    
    Signed-off-by: Mohamed Akram <mohd.ak...@outlook.com>
    Signed-off-by: Christian Brabandt <c...@256bit.org>

diff --git a/runtime/doc/syntax.txt b/runtime/doc/syntax.txt
index a44b6f62b..fe6865e17 100644
--- a/runtime/doc/syntax.txt
+++ b/runtime/doc/syntax.txt
@@ -1,4 +1,4 @@
-*syntax.txt*   For Vim version 9.1.  Last change: 2025 Mar 15
+*syntax.txt*   For Vim version 9.1.  Last change: 2025 Mar 21
 
 
                  VIM REFERENCE MANUAL    by Bram Moolenaar
@@ -3512,25 +3512,25 @@ cases pertain, then the first line of the file is 
examined (ex. looking for
 /bin/sh  /bin/ksh  /bin/bash).  If the first line specifies a shelltype, then
 that shelltype is used.  However some files (ex. .profile) are known to be
 shell files but the type is not apparent.  Furthermore, on many systems sh is
-symbolically linked to "bash" (Linux, Windows+cygwin) or "ksh" (Posix).
+symbolically linked to "bash" (Linux, Windows+cygwin) or "ksh" (POSIX).
 
 One may specify a global default by instantiating one of the following
 variables in your <.vimrc>:
 
    ksh: >
        let g:is_kornshell = 1
-<   posix:  (using this is nearly the same as setting g:is_kornshell to 1) >
+<   posix: (default) >
        let g:is_posix     = 1
 <   bash: >
        let g:is_bash      = 1
-<   sh: (default) Bourne shell >
+<   dash: >
+       let g:is_dash      = 1
+<   sh: Bourne shell >
        let g:is_sh        = 1
 
-<   (dash users should use posix)
-
 If there's no "#! ..." line, and the user hasn't availed himself/herself of a
 default sh.vim syntax setting as just shown, then syntax/sh.vim will assume
-the Bourne shell syntax.  No need to quote RFCs or market penetration
+the POSIX shell syntax.  No need to quote RFCs or market penetration
 statistics in error reports, please -- just select the default version of the
 sh your system uses and install the associated "let..." in your <.vimrc>.
 
diff --git a/runtime/syntax/sh.vim b/runtime/syntax/sh.vim
index 67268cdfe..37d1c0a83 100644
--- a/runtime/syntax/sh.vim
+++ b/runtime/syntax/sh.vim
@@ -7,6 +7,7 @@
 "              2024 Nov 03 by Aliaksei Budavei <0x000c70 AT gmail DOT com> 
(improved bracket expressions, #15941)
 "              2025 Jan 06 add $PS0 to bashSpecialVariables (#16394)
 "              2025 Jan 18 add bash coproc, remove duplicate syn keywords 
(#16467)
+"              2025 Mar 21 update shell capability detection (#16939)
 " Version:             208
 " Former URL:          http://www.drchip.org/astronaut/vim/index.html#SYNTAX_SH
 " For options and settings, please use:      :help ft-sh-syntax
@@ -17,6 +18,9 @@ if exists("b:current_syntax")
   finish
 endif
 
+" Ensure this is set unless we find another shell
+let b:is_sh = 1
+
 " If the shell script itself specifies which shell to use, use it
 if getline(1) =~ '\<ksh\>'
  let b:is_kornshell = 1
@@ -24,55 +28,45 @@ elseif getline(1) =~ '\<bash\>'
  let b:is_bash      = 1
 elseif getline(1) =~ '\<dash\>'
  let b:is_dash      = 1
-elseif !exists("g:is_kornshell") && !exists("g:is_bash") && 
!exists("g:is_posix") && !exists("g:is_sh") && !exists("g:is_dash")
- " user did not specify which shell to use, and
- " the script itself does not specify which shell to use. FYI: /bin/sh is 
ambiguous.
- " Assuming /bin/sh is executable, and if its a link, find out what it links 
to.
- let s:shell = ""
- if executable("/bin/sh")
-  let s:shell = resolve("/bin/sh")
- elseif executable("/usr/bin/sh")
-  let s:shell = resolve("/usr/bin/sh")
- endif
- if     s:shell =~ '\<ksh\>'
-  let b:is_kornshell= 1
- elseif s:shell =~ '\<bash\>'
-  let b:is_bash = 1
- elseif s:shell =~ '\<dash\>'
-  let b:is_dash = 1
- endif
- unlet s:shell
-endif
-
 " handling /bin/sh with is_kornshell/is_sh {{{1
 " b:is_sh will be set when "#! /bin/sh" is found;
 " However, it often is just a masquerade by bash (typically Linux)
 " or kornshell (typically workstations with Posix "sh").
-" So, when the user sets "g:is_bash", "g:is_kornshell",
-" or "g:is_posix", a b:is_sh is converted into b:is_bash/b:is_kornshell,
-" respectively.
-if !exists("b:is_kornshell") && !exists("b:is_bash") && !exists("b:is_dash")
-  if exists("g:is_posix") && !exists("g:is_kornshell")
-   let g:is_kornshell= g:is_posix
+" So, when the user sets "g:is_kornshell", "g:is_bash",
+" "g:is_posix" or "g:is_dash", a b:is_sh is converted into
+" b:is_kornshell/b:is_bash/b:is_posix/b:is_dash, respectively.
+elseif !exists("b:is_kornshell") && !exists("b:is_bash") && 
!exists("b:is_posix") && !exists("b:is_dash")
+ if exists("g:is_kornshell")
+  let b:is_kornshell= 1
+ elseif exists("g:is_bash")
+  let b:is_bash= 1
+ elseif exists("g:is_dash")
+  let b:is_dash= 1
+ elseif exists("g:is_posix")
+  let b:is_posix= 1
+ elseif exists("g:is_sh")
+  let b:is_sh= 1
+ else
+  " user did not specify which shell to use, and
+  " the script itself does not specify which shell to use. FYI: /bin/sh is 
ambiguous.
+  " Assuming /bin/sh is executable, and if its a link, find out what it links 
to.
+  let s:shell = ""
+  if executable("/bin/sh")
+   let s:shell = resolve("/bin/sh")
+  elseif executable("/usr/bin/sh")
+   let s:shell = resolve("/usr/bin/sh")
   endif
-  if exists("g:is_kornshell")
-    let b:is_kornshell= 1
-    if exists("b:is_sh")
-      unlet b:is_sh
-    endif
-  elseif exists("g:is_bash")
-    let b:is_bash= 1
-    if exists("b:is_sh")
-      unlet b:is_sh
-    endif
-  elseif exists("g:is_dash")
-    let b:is_dash= 1
-    if exists("b:is_sh")
-      unlet b:is_sh
-    endif
+  if     s:shell =~ '\<ksh\>'
+   let b:is_kornshell= 1
+  elseif s:shell =~ '\<bash\>'
+   let b:is_bash = 1
+  elseif s:shell =~ '\<dash\>'
+   let b:is_dash = 1
   else
-    let b:is_sh= 1
+   let b:is_posix = 1
   endif
+  unlet s:shell
+ endif
 endif
 
 " if b:is_dash, set b:is_posix too
@@ -80,6 +74,12 @@ if exists("b:is_dash")
  let b:is_posix= 1
 endif
 
+if exists("b:is_kornshell") || exists("b:is_bash")
+ if exists("b:is_sh")
+  unlet b:is_sh
+ endif
+endif
+
 " set up default g:sh_fold_enabled {{{1
 " ================================
 if !exists("g:sh_fold_enabled")

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