Bash c stdin was available to all commands vs. just the first one

src/commands/bash/bash.test.ts

@@ -237,5 +237,16 @@ cat /tmp/test.txt`,
       expect(result.stdout).toBe("line2\n");
       expect(result.exitCode).toBe(0);
     });
+
+    it("should handle command substitution with grep and head in piped bash -c", async () => {
+      const env = new Bash();
+      // This test demonstrates a bug where grep with no matches followed by head
+      // incorrectly passes through the original stdin instead of empty output
+      const result = await env.exec(
+        'echo "test" | bash -c \'RESULT=$(cat | grep "nomatch" | head -1); echo "RESULT=[$RESULT]"\'',
+      );
+      expect(result.stdout).toBe("RESULT=[]\n");
+      expect(result.exitCode).toBe(0);
+    });
   });
 });

src/interpreter/pipeline-execution.ts

@@ -43,12 +43,21 @@ export async function executePipeline(
   for (let i = 0; i < node.commands.length; i++) {
     const command = node.commands[i];
     const isLast = i === node.commands.length - 1;
+    const isFirst = i === 0;
 
     // In a multi-command pipeline, each command runs in a subshell context
     // where $_ starts empty (subshells don't inherit $_ from parent in same way)
     if (isMultiCommandPipeline) {
       // Clear $_ for each pipeline command - they each get fresh subshell context
       ctx.state.lastArg = "";
+
+      // After the first command, clear groupStdin so subsequent commands
+      // only see stdin from the pipeline (even if empty), not the original groupStdin
+      // This prevents commands like head from incorrectly falling back to groupStdin
+      // when they receive empty output from a previous command (e.g., grep with no matches)
+      if (!isFirst) {
+        ctx.state.groupStdin = undefined;
+      }
     }
 
     // Determine if this command runs in a subshell context