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]>

4 files changed, 8 insertions, 6 deletions

diff --git a/test/macos_e2e.swift b/test/macos_e2e.swift
index 5f172f9d..2b0eedad 100644
--- a/test/macos_e2e.swift
+++ b/test/macos_e2e.swift
@@ -1251,7 +1251,7 @@ private func buildWorld(stem: String) throws -> String {
     // sources $HOME/.bashrc (host_io.Shell) — so this is what pins the
     // prompt to `$ ` and keeps the developer's history out of the capture.
     try "PS1='$ '\nHISTFILE=\n".write(toFile: "\(home)/.bashrc", atomically: true, encoding: .utf8)
-    // ...and the config is not empty, because the DEFAULT shell is fish, whose
+    // ...and the config is not empty, because the DEFAULT shell is $SHELL (fish, say), whose
     // prompt carries a hostname and whose greeting carries a version. A golden
     // taken against that is a golden for one machine.
     try "Shell bash\n".write(toFile: "\(pardesConfig)/init", atomically: true, encoding: .utf8)
diff --git a/test/output.zig b/test/output.zig
index 419bcca2..f3e61e93 100644
--- a/test/output.zig
+++ b/test/output.zig
@@ -544,10 +544,11 @@ test "Config prints the startup path and refreshes its one output" {
     for ([_][]const u8{
         "Startup config: " ++ path ++ "\n",
         "Theme orchard\n",
-        "Shell requested (new panes) " ++ config.default_shell ++ " (default)\n",
         "Shell effective (last spawn) (none)\n",
         "Shell pending on\n",
     }) |line| try std.testing.expect(std.mem.indexOf(u8, out.content, line) != null);
+    var default_line: [256]u8 = undefined;
+    try std.testing.expect(std.mem.indexOf(u8, out.content, try std.fmt.bufPrint(&default_line, "Shell requested (new panes) {s} (default)\n", .{config.defaultShell()})) != null);
     try std.testing.expectEqualStrings(config.config_buffer, std.fs.path.basename(out.path));
     try std.testing.expectEqual(panes.Output.Origin{ .cmd = .Config }, out.output.?.from);
 
diff --git a/test/snapshot.zig b/test/snapshot.zig
index 2fa8e89c..c49e7645 100644
--- a/test/snapshot.zig
+++ b/test/snapshot.zig
@@ -562,8 +562,9 @@ fn runScript(arena: std.mem.Allocator, exe_z: [:0]const u8, script_path: []const
     // XDG directory that points outside this per-snapshot fake home.
     const config_home = try std.fmt.allocPrintSentinel(arena, "{s}/.config", .{home}, 0);
     _ = setenv("XDG_CONFIG_HOME", config_home, 1);
-    // ...and it is not empty: the DEFAULT shell is fish, whose prompt carries
-    // a hostname and whose greeting carries a version, so a golden taken
+    // ...and it is not empty: the DEFAULT shell is $SHELL, whatever this
+    // machine's user logs in with (fish's prompt carries a hostname and its
+    // greeting a version), so a golden taken
     // against it would be a golden for this machine. Every script here runs
     // bash — the .bashrc above pins its prompt to `$ ` — and the fish path is
     // covered by host_io.Shell's unit tests instead. This is an ordinary
diff --git a/test/snapshots/shellset.snap b/test/snapshots/shellset.snap
index d2bacf38..ef8b02b9 100644
--- a/test/snapshots/shellset.snap
+++ b/test/snapshots/shellset.snap
@@ -2,8 +2,8 @@
 # here is the case that has to hold on a machine you did not set up: the name
 # resolves to nothing, and the pane opens anyway.
 #
-# That is not a corner — it is the DEFAULT's failure mode. pardes ships
-# `Shell fish` as the default, and a box without fish must still get a working
+# That is not a corner — it is the DEFAULT's failure mode. pardes defaults
+# to $SHELL, which may name a shell the box lacks, and it must still get a working
 # shell rather than a pane whose child dies at exec and shows one EOF. So the
 # lookup falls back, and this is the proof: a name nothing resolves to, and the
 # new pane still greets and lists like any other.