The default Shell is $SHELL, else /bin/sh

With no Shell set, pardes ran fish, which a box may not have and a user
may not use. The default is now the user's login shell ($SHELL), else
/bin/sh; an explicit Shell still overrides it. The tests pin their own
shell (snapshots bash, the python suites /bin/sh or bash) and pass with
SHELL unset; the Config report and docs say where the default comes from.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

5 files changed, 15 insertions, 7 deletions

diff --git a/src/Output.zig b/src/Output.zig
index 2f462fd1..ee932768 100644
--- a/src/Output.zig
+++ b/src/Output.zig
@@ -665,7 +665,7 @@ pub fn openConfig(p: *Pardes, id: usize) !void {
         .startup_config_path = p.opts.startup_config_path,
         .platform = @tagName(pardes.platform),
         .theme_name = p.theme().name,
-        .compiled_default_shell = config.default_shell,
+        .compiled_default_shell = config.defaultShell(),
         .gui_shader_source_mode = if (gui_shader_source_mode) |mode|
             mode.label()
         else
diff --git a/src/builtins.zig b/src/builtins.zig
index 8c6228c0..f7104a8c 100644
--- a/src/builtins.zig
+++ b/src/builtins.zig
@@ -399,7 +399,7 @@ test "Ligatures is a setting only in the shell that shapes text" {
         .startup_config_path = null,
         .platform = @tagName(pardes.platform),
         .theme_name = p.theme().name,
-        .compiled_default_shell = config.default_shell,
+        .compiled_default_shell = config.defaultShell(),
         .hover_delay_frames = null,
         .native_images = false,
         .capabilities = capabilities,
diff --git a/src/config.zig b/src/config.zig
index fb38e4dd..a59fe18d 100644
--- a/src/config.zig
+++ b/src/config.zig
@@ -345,7 +345,15 @@ test "wheel drift guard" {
     try std.testing.expect(wheelTick(&g, false));
 }
 
-pub const default_shell = "fish";
+/// The shell when no `Shell` is set: the user's login shell ($SHELL), as a
+/// terminal would start, else /bin/sh. An explicit `Shell` overrides it.
+pub fn defaultShell() []const u8 {
+    if (comptime pardes.hosted) if (std.c.getenv("SHELL")) |s| {
+        const shell = std.mem.span(s);
+        if (shell.len > 0) return shell;
+    };
+    return "/bin/sh";
+}
 
 // Minimum file gutter width, including the space after the line number.
 pub const PREFIX_W: u16 = 5;
@@ -1364,7 +1372,7 @@ pub const Runtime = struct {
             try out.writeAll("Startup config: no per-user config path\n");
         try out.print(
             "Platform: {s}\n" ++
-                "Compiled default shell: {s}\n",
+                "Default shell ($SHELL, else /bin/sh): {s}\n",
             .{ context.platform, context.compiled_default_shell },
         );
         if (context.gui_shader_source_mode) |mode|
@@ -1719,7 +1727,7 @@ pub const Runtime = struct {
             "TaglineSize 82\n",
             "Startup config: /tmp/pardes/init\n",
             "Platform: gui\n",
-            "Compiled default shell: /bin/sh\n",
+            "Default shell ($SHELL, else /bin/sh): /bin/sh\n",
             "GUI shader source: live GLSL compiled during this build\n",
             "Look hover delay: 18 frames\n",
             "Native images: on\n",
diff --git a/src/ninep/ctl.zig b/src/ninep/ctl.zig
index 8a35c381..0ab63fbb 100644
--- a/src/ninep/ctl.zig
+++ b/src/ninep/ctl.zig
@@ -399,7 +399,7 @@ pub fn writeSettingsOf(p: *Pardes, s: *const config.Runtime, w: *std.Io.Writer)
             .theme => try w.print("{s} {s}\n", .{ word, if (s == &p.settings) p.theme().name else pardes.themes[s.theme].name }),
             .shell => {
                 const chosen = s.shell.requested.get();
-                try w.print("{s} {s}\n", .{ word, if (chosen.len > 0) chosen else config.default_shell });
+                try w.print("{s} {s}\n", .{ word, if (chosen.len > 0) chosen else config.defaultShell() });
             },
             .font => {
                 const requested = s.font.requested_name.get();
diff --git a/src/pardes.zig b/src/pardes.zig
index a0b11566..f2791e33 100644
--- a/src/pardes.zig
+++ b/src/pardes.zig
@@ -4925,7 +4925,7 @@ pub const Pardes = struct {
 
     pub fn shellBin(p: *const Pardes) []const u8 {
         const selected = p.settings.shell.requested.get();
-        return if (selected.len == 0) config.default_shell else selected;
+        return if (selected.len == 0) config.defaultShell() else selected;
     }
 
     pub fn acknowledgeShell(p: *Pardes, id: usize, executable: []const u8, prompt_marks: bool) void {