From 481807255ad669cb25f421e59afdc68c124c39db Mon Sep 17 00:00:00 2001
From: Chris Wanstrath <chris@ozmm.org>
Date: Thu, 12 Mar 2026 16:06:33 -0700
Subject: [PATCH] Isolate per-command output from background procs

---
 src/run.test.ts | 18 ++++++++++++++++++
 src/run.ts      | 17 ++++++++++++++---
 2 files changed, 32 insertions(+), 3 deletions(-)

diff --git a/src/run.test.ts b/src/run.test.ts
index a06f994..472c265 100644
--- a/src/run.test.ts
+++ b/src/run.test.ts
@@ -62,6 +62,24 @@ describe("runFile", () => {
     }
   })
 
+  test("background process output does not leak into subsequent commands", async () => {
+    const file = makeFile([
+      // Start a background process that writes to stdout after a delay
+      { command: "{ sleep 0.1; echo LEAKED; } &" },
+      // Wait long enough for the background output to appear
+      { command: "sleep 0.3; echo clean" },
+    ])
+    const result = await runFile(file, defaultOpts)
+    try {
+      // The background command itself should have no output
+      expect(result.results[0]?.actual).toEqual([])
+      // The sleep command should only see its own output, not the background "LEAKED"
+      expect(result.results[1]?.actual).toEqual(["clean"])
+    } finally {
+      await cleanupTmpDir(result.tmpDir)
+    }
+  })
+
   test("cleans up multiple backgrounded processes", async () => {
     const file = makeFile([
       { command: "sleep 300 >/dev/null 2>&1 & P1=$!; sleep 300 >/dev/null 2>&1 & P2=$!; echo $P1 $P2" },
diff --git a/src/run.ts b/src/run.ts
index 378d08f..a1da05a 100644
--- a/src/run.ts
+++ b/src/run.ts
@@ -58,9 +58,10 @@ function buildScript(commands: Command[], sentinel: string, verbose: boolean): s
 
   if (verbose) {
     // Save original stderr to fd 3 before merging stderr into stdout
-    lines.push("exec 3>&2 2>&1")
+    // Save the pipe on fd 9 so we can restore after each command
+    lines.push("exec 3>&2 2>&1 9>&1")
   } else {
-    lines.push("exec 2>&1")
+    lines.push("exec 2>&1 9>&1")
   }
 
   for (let i = 0; i < commands.length; i++) {
@@ -68,11 +69,21 @@ function buildScript(commands: Command[], sentinel: string, verbose: boolean): s
     if (verbose) {
       lines.push(`printf '${VERBOSE_MARKER}${i}\\n' >&3`)
     }
+    // Redirect stdout+stderr to a temp file so background processes
+    // from previous commands can't pollute this command's output.
+    // Background processes keep their fd pointing at the old temp file,
+    // which becomes orphaned after rm — their output goes nowhere.
+    lines.push(`__shout_out=$(mktemp)`)
+    lines.push(`exec 1>"$__shout_out" 2>&1`)
     lines.push(cmd.command)
+    lines.push(`__shout_ec=$?`)
+    lines.push(`exec 1>&9 2>&1`)
+    lines.push(`cat "$__shout_out"`)
+    lines.push(`rm -f "$__shout_out"`)
     // Sentinel: printf to avoid echo interpretation issues
     // Format: __SHOUT_SENTINEL_<exitcode>_<index>__
     lines.push(
-      `printf '\\n${sentinel}%s_${i}__\\n' "$?"`,
+      `printf '\\n${sentinel}%s_${i}__\\n' "$__shout_ec"`,
     )
   }