shell: expand $RANDOM, $UID, $PPID, $SECONDS, $IFS, $OSTYPE and friends

src/runtime/shell/interpreter.rs

@@ -588,6 +588,7 @@ impl Interpreter {
                 __cwd: cwd_arr,
                 cwd_fd,
                 async_pids: SmolList::default(),
+                start_instant: std::time::Instant::now(),
             }),
             root_io: JsCell::new(IO {
                 stdin: crate::shell::io::InKind::Fd(stdin_reader),
@@ -1793,6 +1794,9 @@ pub struct ShellExecEnv {
     pub __cwd: Vec<u8>,
     pub cwd_fd: Fd,
     pub async_pids: SmolList<PidT, 4>,
+    /// For `$SECONDS`. Copied (not reset) by `dupe_for_subshell` so a subshell
+    /// sees the parent's elapsed time, matching bash.
+    pub start_instant: std::time::Instant,
 }
 
 pub enum Bufio {
@@ -1958,6 +1962,7 @@ impl ShellExecEnv {
             __cwd: self.__cwd.clone(),
             cwd_fd: dupedfd,
             async_pids: SmolList::default(),
+            start_instant: self.start_instant,
         });
         Ok(bun_core::heap::into_raw(duped))
     }
@@ -2167,6 +2172,93 @@ impl ShellExecEnv {
                 })
             })
     }
+
+    /// Fallback for bash's dynamic / auto-initialized shell variables when
+    /// `label` was not found in `shell_env` or `export_env`. Appends the
+    /// expansion to `out` and returns `true` on a hit. A user assignment
+    /// (which lands in one of the env maps) therefore shadows these.
+    pub fn append_special_var(&self, label: &[u8], out: &mut Vec<u8>) -> bool {
+        use std::io::Write as _;
+        match label {
+            b"RANDOM" => {
+                // bash: 15-bit value, fresh on every reference.
+                let n = (bun_core::fast_random() & 0x7fff) as u32;
+                let _ = write!(out, "{n}");
+            }
+            b"SECONDS" => {
+                let secs = self.start_instant.elapsed().as_secs();
+                let _ = write!(out, "{secs}");
+            }
+            b"IFS" => out.extend_from_slice(b" \t\n"),
+            b"SHLVL" => out.push(b'1'),
+            b"LINENO" => out.push(b'0'),
+            b"OSTYPE" => out.extend_from_slice(bash_ostype()),
+            b"HOSTNAME" => append_hostname(out),
+            #[cfg(unix)]
+            b"UID" => {
+                let _ = write!(out, "{}", bun_sys::c::getuid());
+            }
+            #[cfg(unix)]
+            b"EUID" => {
+                let _ = write!(out, "{}", bun_sys::c::geteuid());
+            }
+            #[cfg(unix)]
+            b"GROUPS" => {
+                let _ = write!(out, "{}", bun_sys::c::getgid());
+            }
+            #[cfg(unix)]
+            b"PPID" => {
+                // SAFETY: getppid has no preconditions and never fails.
+                let _ = write!(out, "{}", unsafe { libc::getppid() });
+            }
+            #[cfg(windows)]
+            b"PPID" => {
+                // SAFETY: uv_os_getppid has no preconditions.
+                let _ = write!(out, "{}", unsafe { bun_libuv_sys::uv_os_getppid() });
+            }
+            _ => return false,
+        }
+        true
+    }
+}
+
+/// bash's `$OSTYPE` for the host target. bash derives this from
+/// `config.guess`; these are the strings scripts match against.
+const fn bash_ostype() -> &'static [u8] {
+    if cfg!(target_os = "macos") {
+        b"darwin"
+    } else if cfg!(target_os = "android") {
+        b"linux-android"
+    } else if cfg!(target_env = "musl") {
+        b"linux-musl"
+    } else if cfg!(target_os = "linux") {
+        b"linux-gnu"
+    } else if cfg!(target_os = "freebsd") {
+        b"freebsd"
+    } else if cfg!(windows) {
+        b"msys"
+    } else {
+        b"unknown"
+    }
+}
+
+#[cfg(unix)]
+fn append_hostname(out: &mut Vec<u8>) {
+    let mut buf = [0u8; 256];
+    if bun_sys::posix::gethostname(&mut buf).is_ok() {
+        out.extend_from_slice(bun_core::slice_to_nul(&buf));
+    }
+}
+
+#[cfg(windows)]
+fn append_hostname(out: &mut Vec<u8>) {
+    let mut buf = [0u8; 256];
+    let mut size: usize = buf.len();
+    // SAFETY: `buf` is valid for `size` writes; `size` is in-out. libuv
+    // handles WSAStartup itself, so this works before any socket init.
+    if unsafe { bun_libuv_sys::uv_os_gethostname(buf.as_mut_ptr().cast(), &mut size) } == 0 {
+        out.extend_from_slice(&buf[..size]);
+    }
 }
 
 // ────────────────────────────────────────────────────────────────────────────

src/runtime/shell/states/Expansion.rs

@@ -485,14 +485,18 @@ impl Expansion {
                 *has_quoted_empty = true;
             }
             ast::SimpleAtom::Var(label) => {
-                // Spec `expandVar`: shell_env first, then export_env, else "".
+                // Spec `expandVar`: shell_env first, then export_env, then the
+                // dynamic-special-variable fallback (`$RANDOM`, `$UID`, ...),
+                // else "".
                 let key = EnvStr::init_slice(label);
                 if let Some(v) = shell.shell_env.get(key) {
                     out.extend_from_slice(v.slice());
                     v.deref();
                 } else if let Some(v) = shell.export_env.get(EnvStr::init_slice(label)) {
                     out.extend_from_slice(v.slice());
                     v.deref();
+                } else {
+                    shell.append_special_var(label, out);
                 }
             }
             ast::SimpleAtom::VarArgv(int) => {

test/js/bun/shell/bunshell.test.ts

@@ -9,6 +9,7 @@ import { afterAll, beforeAll, describe, expect, it, test } from "bun:test";
 import { chmodSync, mkdirSync } from "fs";
 import { mkdir, rm, stat } from "fs/promises";
 import { bunExe, isPosix, isWindows, runWithErrorPromise, tempDir, tempDirWithFiles, tmpdirSync } from "harness";
+import { hostname as osHostname } from "node:os";
 import { join, sep } from "path";
 import { createTestBuilder, sortedShellOutput } from "./util";
 const TestBuilder = createTestBuilder(import.meta.path);
@@ -1330,6 +1331,106 @@ describe("deno_task", () => {
       .runAsTest("exported vars 2");
   });
 
+  // bash's auto-initialized / dynamic shell variables: these used to fall
+  // through the env-map lookup and expand to the empty string.
+  describe("special shell variables", () => {
+    // `bunEnv` above is `{ ...process.env, ... }`, so HOSTNAME / SHLVL /
+    // OSTYPE may already be inherited. Strip every name under test so the
+    // fallback path is what's exercised.
+    const envNoSpecials: Record<string, string> = { ...bunEnv };
+    for (const k of [
+      "RANDOM",
+      "UID",
+      "EUID",
+      "GROUPS",
+      "PPID",
+      "SECONDS",
+      "LINENO",
+      "IFS",
+      "SHLVL",
+      "HOSTNAME",
+      "OSTYPE",
+    ]) {
+      delete envNoSpecials[k];
+    }
+
+    test("$RANDOM is a fresh 15-bit integer on each reference", async () => {
+      const out = await $`${{ raw: `printf '%s %s %s' "$RANDOM" "$RANDOM" "$RANDOM"` }}`.env(envNoSpecials).text();
+      const parts = out.split(" ");
+      expect(parts).toHaveLength(3);
+      for (const p of parts) {
+        expect(p).toMatch(/^\d+$/);
+        const n = Number(p);
+        expect(n).toBeGreaterThanOrEqual(0);
+        expect(n).toBeLessThanOrEqual(32767);
+      }
+      // Three independent 15-bit draws: P(all equal) is 2^-30.
+      expect(new Set(parts).size).toBeGreaterThan(1);
+    });
+
+    test("tmp.$RANDOM.$RANDOM yields distinct names across runs", async () => {
+      const run = () => $`${{ raw: `n=tmp.$RANDOM.$RANDOM; printf '%s' "$n"` }}`.env(envNoSpecials).text();
+      const [a, b] = await Promise.all([run(), run()]);
+      expect(a).toMatch(/^tmp\.\d+\.\d+$/);
+      expect(b).toMatch(/^tmp\.\d+\.\d+$/);
+      expect(a).not.toBe(b);
+      expect(a).not.toBe("tmp..");
+    });
+
+    test("$IFS defaults to <space><tab><newline>", async () => {
+      expect(await $`${{ raw: `printf '[%s]' "$IFS"` }}`.env(envNoSpecials).text()).toBe("[ \t\n]");
+    });
+
+    test("$SECONDS starts at 0", async () => {
+      expect(await $`${{ raw: `printf '%s' "$SECONDS"` }}`.env(envNoSpecials).text()).toBe("0");
+    });
+
+    test("$SHLVL defaults to 1 when unset in the environment", async () => {
+      expect(await $`${{ raw: `printf '%s' "$SHLVL"` }}`.env(envNoSpecials).text()).toBe("1");
+    });
+
+    test("$LINENO expands to a number", async () => {
+      expect(await $`${{ raw: `printf '%s' "$LINENO"` }}`.env(envNoSpecials).text()).toMatch(/^\d+$/);
+    });
+
+    test("$OSTYPE is a non-empty platform string", async () => {
+      const out = await $`${{ raw: `printf '%s' "$OSTYPE"` }}`.env(envNoSpecials).text();
+      expect(out.length).toBeGreaterThan(0);
+      if (process.platform === "darwin") expect(out).toStartWith("darwin");
+      else if (process.platform === "linux") expect(out).toStartWith("linux");
+      else if (process.platform === "win32") expect(out).toBe("msys");
+    });
+
+    test("$HOSTNAME matches os.hostname()", async () => {
+      expect(await $`${{ raw: `printf '%s' "$HOSTNAME"` }}`.env(envNoSpecials).text()).toBe(osHostname());
+    });
+
+    test("$PPID is the parent process id", async () => {
+      const out = await $`${{ raw: `printf '%s' "$PPID"` }}`.env(envNoSpecials).text();
+      expect(out).toMatch(/^\d+$/);
+      // The shell runs in-process, so its PPID is this process's PPID.
+      expect(out).toBe(String(process.ppid));
+    });
+
+    if (isPosix) {
+      test("$UID / $EUID / $GROUPS match the real process credentials", async () => {
+        const out = await $`${{ raw: `printf '%s %s %s' "$UID" "$EUID" "$GROUPS"` }}`.env(envNoSpecials).text();
+        expect(out).toBe(`${process.getuid!()} ${process.geteuid!()} ${process.getgid!()}`);
+      });
+    }
+
+    test("user assignment shadows the special-variable fallback", async () => {
+      const out = await $`${{ raw: `RANDOM=pinned; IFS=:; printf '%s %s' "$RANDOM" "$IFS"` }}`
+        .env(envNoSpecials)
+        .text();
+      expect(out).toBe("pinned :");
+    });
+
+    test("unknown variable still expands to the empty string", async () => {
+      expect(await $`${{ raw: `printf '[%s]' "$DEFINITELY_NOT_SET_123"` }}`.env(envNoSpecials).text()).toBe("[]");
+    });
+  });
+
   describe("pipeline", async () => {
     TestBuilder.command`echo 1 | BUN_TEST_VAR=1 ${BUN} -e 'process.stdin.pipe(process.stdout)'`
       .stdout("1\n")