--- a/doc/interpreter/system.txi	Tue Jul 11 19:43:03 2023 +0200
+++ b/doc/interpreter/system.txi	Wed May 31 16:29:09 2023 -0400
@@ -425,6 +425,8 @@
 
 @DOCSTRING(getenv)
 
+@DOCSTRING(isenv)
+
 @DOCSTRING(setenv)
 
 @DOCSTRING(unsetenv)

--- a/etc/NEWS.9.md	Tue Jul 11 19:43:03 2023 +0200
+++ b/etc/NEWS.9.md	Wed May 31 16:29:09 2023 -0400
@@ -122,6 +122,7 @@
 
 ### Alphabetical list of new functions added in Octave 9
 
+* `isenv`
 * `isuniform`
 * `tensorprod`
 

--- a/libinterp/corefcn/sysdep.cc	Tue Jul 11 19:43:03 2023 +0200
+++ b/libinterp/corefcn/sysdep.cc	Wed May 31 16:29:09 2023 -0400
@@ -755,7 +755,7 @@
 
 @noindent
 returns a string containing the value of your path.
-@seealso{setenv, unsetenv}
+@seealso{setenv, unsetenv, isenv}
 @end deftypefn */)
 {
   if (args.length () != 1)
@@ -770,6 +770,43 @@
 %!assert (ischar (getenv ("OCTAVE_HOME")))
 */
 
+DEFUN (isenv, args, ,
+       doc: /* -*- texinfo -*-
+@deftypefn {} {@var{val} =} isenv (@var{var})
+Check if the environment variable @var{var} exists.
+
+This function returns true if an environment variable with the name @var{var}
+exists.  Otherwise, it returns false.
+
+For example,
+
+@example
+tf = isenv ("PATH")
+@end example
+
+@noindent
+returns true if an environment variable with the name @qcode{"PATH"} exists.
+@seealso{getenv, setenv, unsetenv}
+@end deftypefn */)
+{
+  if (args.length () != 1)
+    print_usage ();
+
+  std::string name = args(0).xstring_value ("isenv: VAR must be a string");
+
+  return ovl (sys::env::isenv (name));
+}
+
+/*
+%!test
+%! setenv ("dummy_variable_that_cannot_matter", "foobar");
+%! assert (isenv ("dummy_variable_that_cannot_matter"), true);
+%! unsetenv ("dummy_variable_that_cannot_matter");
+%! assert (isenv ("dummy_variable_that_cannot_matter"), false);
+
+%!error <VAR must be a string> isenv (struct ())
+*/
+
 DEFUN (setenv, args, ,
        doc: /* -*- texinfo -*-
 @deftypefn  {} {} setenv ("@var{var}", @var{value})
@@ -782,7 +819,7 @@
 
 Programming Note: @code{putenv} is an alias for @code{setenv} and can be used
 interchangeably.
-@seealso{unsetenv, getenv}
+@seealso{unsetenv, getenv, isenv}
 @end deftypefn */)
 {
   int nargin = args.length ();
@@ -818,7 +855,7 @@
 
 Return 0 if the variable was deleted, or did not exist, and -1 if an error
 occurred.
-@seealso{setenv, getenv}
+@seealso{setenv, getenv, isenv}
 @end deftypefn */)
 {
   if (args.length () != 1)

--- a/liboctave/system/lo-sysdep.cc	Tue Jul 11 19:43:03 2023 +0200
+++ b/liboctave/system/lo-sysdep.cc	Wed May 31 16:29:09 2023 -0400
@@ -705,6 +705,18 @@
 #endif
 }
 
+bool
+isenv_wrapper (const std::string& name)
+{
+#if defined (OCTAVE_USE_WINDOWS_API)
+  std::wstring wname = u8_to_wstring (name);
+  wchar_t *env = _wgetenv (wname.c_str ());
+#else
+  char *env = ::getenv (name.c_str ());
+#endif
+  return env != 0;
+}
+
 std::wstring
 u8_to_wstring (const std::string& utf8_string)
 {

--- a/liboctave/system/lo-sysdep.h	Tue Jul 11 19:43:03 2023 +0200
+++ b/liboctave/system/lo-sysdep.h	Wed May 31 16:29:09 2023 -0400
@@ -88,6 +88,8 @@
 
 extern OCTAVE_API std::string getenv_wrapper (const std::string&);
 
+extern OCTAVE_API bool isenv_wrapper (const std::string&);
+
 extern OCTAVE_API int unsetenv_wrapper (const std::string&);
 
 extern OCTAVE_API std::wstring u8_to_wstring (const std::string&);

--- a/liboctave/system/oct-env.cc	Tue Jul 11 19:43:03 2023 +0200
+++ b/liboctave/system/oct-env.cc	Wed May 31 16:29:09 2023 -0400
@@ -299,6 +299,12 @@
          ? s_instance->do_getenv (name) : "";
 }
 
+bool
+env::isenv (const std::string& name)
+{
+  return isenv_wrapper (name);
+}
+
 void
 env::putenv (const std::string& name, const std::string& value)
 {

--- a/liboctave/system/oct-env.h	Tue Jul 11 19:43:03 2023 +0200
+++ b/liboctave/system/oct-env.h	Wed May 31 16:29:09 2023 -0400
@@ -80,6 +80,8 @@
 
   static std::string getenv (const std::string& name);
 
+  static bool isenv (const std::string& name);
+
   static void putenv (const std::string& name, const std::string& value);
 
   static bool have_x11_display ();