From 9eae49a2b5a993f23d148e8bc499ae55cf405b73 Mon Sep 17 00:00:00 2001
From: Cameron Taggart <cataggar@microsoft.com>
Date: Sun, 16 Aug 2026 18:05:40 +0100
Subject: [PATCH] fix(parser): honor backslash-newline line continuations in
 here-documents

In a here-document whose delimiter is unquoted, bash removes a
backslash-newline pair outright and joins the two lines; a backslash is
only literal there when it precedes `$`, a backtick, or another
backslash. `heredoc_escape_sequence` covered those three characters but
not newline, so the pair fell through to `heredoc_literal_text` and both
the backslash and the newline were emitted verbatim.

Add an explicit line-continuation rule and exclude it from the literal
text rule. A quoted delimiter suppresses expansion entirely, so that path
is untouched and continues to preserve the backslash, matching bash.

Before:                       After (and bash):
  $ cat <<EOF                   $ cat <<EOF
  > alpha \                     > alpha \
  > beta                        > beta
  > EOF                         > EOF
  alpha \                       alpha beta
  beta

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
---
 brush-parser/src/word.rs                 |  9 +++++-
 brush-shell/tests/cases/compat/here.yaml | 36 +++++++++++++++++++++++-
 2 files changed, 43 insertions(+), 2 deletions(-)

diff --git a/brush-parser/src/word.rs b/brush-parser/src/word.rs
index 99c4195f6..8819ec0fc 100644
--- a/brush-parser/src/word.rs
+++ b/brush-parser/src/word.rs
@@ -872,14 +872,21 @@ peg::parser! {
             legacy_arithmetic_expansion() /
             command_substitution() /
             parameter_expansion() /
+            heredoc_line_continuation() /
             heredoc_escape_sequence() /
             heredoc_literal_text()
 
+        // In a here-document whose delimiter is unquoted, a backslash-newline pair is a
+        // line continuation: it is removed outright, joining the two lines. (A delimiter
+        // that *is* quoted suppresses expansion altogether, so this rule never sees it.)
+        rule heredoc_line_continuation() -> WordPiece =
+            "\\\n" { WordPiece::Text(String::new()) }
+
         rule heredoc_escape_sequence() -> WordPiece =
             s:$("\\" ['$' | '`' | '\\']) { WordPiece::EscapeSequence(s.to_owned()) }
 
         rule heredoc_literal_text() -> WordPiece =
-            s:$((!heredoc_escape_sequence() !dollar_sign_word_piece() [^'`'])+) {
+            s:$((!heredoc_line_continuation() !heredoc_escape_sequence() !dollar_sign_word_piece() [^'`'])+) {
                 WordPiece::Text(s.to_owned())
             }
 
diff --git a/brush-shell/tests/cases/compat/here.yaml b/brush-shell/tests/cases/compat/here.yaml
index fe6ecb8fe..32982e0ff 100644
--- a/brush-shell/tests/cases/compat/here.yaml
+++ b/brush-shell/tests/cases/compat/here.yaml
@@ -306,4 +306,38 @@ cases:
         i=$((i + 1))
       done
       out=$(cat <<<"$payload")
-      echo "${#out}"
\ No newline at end of file
+      echo "${#out}"
+
+  - name: "Here doc with line continuation"
+    stdin: |
+      cat <<EOF
+      alpha \
+      beta
+      EOF
+
+  - name: "Here doc with line continuation and quoted tag"
+    stdin: |
+      cat <<'EOF'
+      alpha \
+      beta
+      EOF
+
+  - name: "Here doc with line continuation before expansion"
+    stdin: |
+      v=world
+      cat <<EOF
+      hello \
+      $v
+      EOF
+
+  - name: "Here doc escapes alongside line continuation"
+    stdin: |
+      v=world
+      cat <<EOF
+      plain $v
+      dollar \$v
+      backslash \\ end
+      backtick \` end
+      cont \
+      joined
+      EOF
\ No newline at end of file