diff --git a/src/os/env.zig b/src/os/env.zig
new file mode 100644
index 000000000..d6c2970c5
--- /dev/null
+++ b/src/os/env.zig
@@ -0,0 +1,48 @@
+const std = @import("std");
+const builtin = @import("builtin");
+const Allocator = std.mem.Allocator;
+
+/// Append a value to an environment variable such as PATH.
+/// The returned value is always allocated so it must be freed.
+pub fn appendEnv(
+    alloc: Allocator,
+    current: []const u8,
+    value: []const u8,
+) ![]u8 {
+    // If there is no prior value, we return it as-is
+    if (current.len == 0) return try alloc.dupe(u8, value);
+
+    // Otherwise we must prefix.
+    const sep = switch (builtin.os.tag) {
+        .windows => ";",
+        else => ":",
+    };
+
+    return try std.fmt.allocPrint(alloc, "{s}{s}{s}", .{
+        current,
+        sep,
+        value,
+    });
+}
+
+test "appendEnv empty" {
+    const testing = std.testing;
+    const alloc = testing.allocator;
+
+    const result = try appendEnv(alloc, "", "foo");
+    defer alloc.free(result);
+    try testing.expectEqualStrings(result, "foo");
+}
+
+test "appendEnv existing" {
+    const testing = std.testing;
+    const alloc = testing.allocator;
+
+    const result = try appendEnv(alloc, "a:b", "foo");
+    defer alloc.free(result);
+    if (builtin.os.tag == .windows) {
+        try testing.expectEqualStrings(result, "a:b;foo");
+    } else {
+        try testing.expectEqualStrings(result, "a:b:foo");
+    }
+}
diff --git a/src/os/main.zig b/src/os/main.zig
index b80c116e6..a9df1fca6 100644
--- a/src/os/main.zig
+++ b/src/os/main.zig
@@ -1,6 +1,7 @@
 //! The "os" package contains utilities for interfacing with the operating
 //! system.
 
+pub usingnamespace @import("env.zig");
 pub usingnamespace @import("file.zig");
 pub usingnamespace @import("flatpak.zig");
 pub usingnamespace @import("homedir.zig");