From 310d7b66b33d20c5380a77dc1ffcd4615451ac21 Mon Sep 17 00:00:00 2001
From: Jules Macret <jules.macret@datadoghq.com>
Date: Wed, 15 Apr 2026 13:45:56 +0200
Subject: [PATCH] fix(cat): suppress broken-pipe error when piped into head
 (#163)

When cat pipes into head, head exits after reading enough lines and
closes the pipe. cat's next write gets EPIPE, which was being printed
to stderr. In bash, SIGPIPE silently terminates the writer. Add
IsBrokenPipe helper and use it in cat to silently exit on EPIPE.

Closes #163

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
---
 builtins/builtins.go                          | 10 +++++++
 builtins/cat/cat.go                           |  3 +++
 .../pipe/basic/cat_head_no_broken_pipe.yaml   | 26 +++++++++++++++++++
 3 files changed, 39 insertions(+)
 create mode 100644 tests/scenarios/shell/pipe/basic/cat_head_no_broken_pipe.yaml

diff --git a/builtins/builtins.go b/builtins/builtins.go
index 23328eb5..332948aa 100644
--- a/builtins/builtins.go
+++ b/builtins/builtins.go
@@ -7,11 +7,13 @@ package builtins
 
 import (
 	"context"
+	"errors"
 	"fmt"
 	"io"
 	"io/fs"
 	"os"
 	"sort"
+	"syscall"
 	"time"
 
 	"github.com/spf13/pflag"
@@ -199,6 +201,14 @@ func (c *CallContext) Errf(format string, a ...any) {
 	fmt.Fprintf(c.Stderr, format, a...)
 }
 
+// IsBrokenPipe reports whether err is a broken-pipe (EPIPE) error,
+// which occurs when writing to a pipe whose read end has been closed.
+// In bash this triggers SIGPIPE which silently terminates the writer;
+// builtins should use this to suppress error messages on pipe closure.
+func IsBrokenPipe(err error) bool {
+	return err != nil && errors.Is(err, syscall.EPIPE)
+}
+
 // FileID is a comparable file identity for cycle detection.
 // On Unix: device + inode. On Windows: volume serial + file index.
 // Used as map key for visited-set tracking.
diff --git a/builtins/cat/cat.go b/builtins/cat/cat.go
index 9b86473b..79530bd8 100644
--- a/builtins/cat/cat.go
+++ b/builtins/cat/cat.go
@@ -162,6 +162,9 @@ func registerFlags(fs *builtins.FlagSet) builtins.HandlerFunc {
 				err = catRaw(ctx, callCtx, file)
 			}
 			if err != nil {
+				if builtins.IsBrokenPipe(err) {
+					break
+				}
 				name := file
 				if file == "-" {
 					name = "standard input"
diff --git a/tests/scenarios/shell/pipe/basic/cat_head_no_broken_pipe.yaml b/tests/scenarios/shell/pipe/basic/cat_head_no_broken_pipe.yaml
new file mode 100644
index 00000000..17f910ed
--- /dev/null
+++ b/tests/scenarios/shell/pipe/basic/cat_head_no_broken_pipe.yaml
@@ -0,0 +1,26 @@
+description: cat piped into head must not emit a broken-pipe error on stderr.
+setup:
+  files:
+    - path: big.txt
+      content: |+
+        line 1
+        line 2
+        line 3
+        line 4
+        line 5
+        line 6
+        line 7
+        line 8
+        line 9
+        line 10
+input:
+  allowed_paths: ["$DIR"]
+  script: |+
+    cat big.txt | head -n 3
+expect:
+  stdout: |+
+    line 1
+    line 2
+    line 3
+  stderr: |+
+  exit_code: 0