head,tail: quote name in file headers appropriately

* src/tail.c (write_header): Use quotef() to quote
file names when needed.
* src/head.c (write_header): Likewise.
* tests/head/quote-headers.sh: Add a new test.
* tests/tail/quote-headers.sh: Add a new test.
* tests/local.mk: Reference the new test.
Reported by Michał Majchrowicz.

NEWS

@@ -8,6 +8,9 @@ GNU coreutils NEWS                                    -*- outline -*-
   mistakenly exit with a nonzero status.
   [This bug was present in "the beginning".]
 
+  'head' and 'tail' now quote names in file headers when needed.
+  [This bug was present in "the beginning".]
+
   'pinky -l' no longer no longer prints output in the incorrect order when
   standard output is fully buffered, e.g., when redirected to a file.
   [bug introduced in coreutils-9.10]

src/head.c

@@ -180,7 +180,7 @@ write_header (char const *filename)
 {
   static bool first_file = true;
 
-  printf ("%s==> %s <==\n", (first_file ? "" : "\n"), filename);
+  printf ("%s==> %s <==\n", (first_file ? "" : "\n"), quotef (filename));
   first_file = false;
 }
 

src/tail.c

@@ -461,7 +461,7 @@ write_header (char const *prettyname)
 {
   static bool first_file = true;
 
-  printf ("%s==> %s <==\n", first_file ? "" : "\n", prettyname);
+  printf ("%s==> %s <==\n", first_file ? "" : "\n", quotef (prettyname));
   first_file = false;
 }
 

tests/head/quote-headers.sh

@@ -0,0 +1,38 @@
+#!/bin/sh
+# Ensure head quotes file names appropriately in headers
+# Before coreutils 9.12 'head ...' would not quote problematic file names.
+
+# Copyright (C) 2026 Free Software Foundation, Inc.
+
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+. "${srcdir=.}/tests/init.sh"; path_prepend_ ./src
+print_ver_ head
+
+NL='
+'
+
+touch "$NL" normal || framework_failure_
+
+head -n1 "$NL" normal >out || fail=1
+
+cat<<\EOF>exp || framework_failure_
+==> ''$'\n' <==
+
+==> normal <==
+EOF
+
+compare exp out || fail=1
+
+Exit $fail

tests/local.mk

@@ -197,6 +197,7 @@ all_tests =					\
   tests/tail/inotify-rotate-resources.sh	\
   tests/tail/inotify-dir-recreate.sh		\
   tests/tail/inotify-only-regular.sh		\
+  tests/tail/quote-headers.sh			\
   tests/chmod/no-x.sh				\
   tests/chgrp/basic.sh				\
   tests/rm/dangling-symlink.sh			\
@@ -368,6 +369,7 @@ all_tests =					\
   tests/head/head-n0.sh				\
   tests/head/head-pos.sh			\
   tests/head/head-write-error.sh		\
+  tests/head/quote-headers.sh			\
   tests/misc/kill.sh				\
   tests/cksum/b2sum.sh				\
   tests/cksum/md5sum.pl				\

tests/tail/quote-headers.sh

@@ -0,0 +1,38 @@
+#!/bin/sh
+# Ensure tail quotes file names appropriately in headers
+# Before coreutils 9.12 'tail ...' would not quote problematic file names.
+
+# Copyright (C) 2026 Free Software Foundation, Inc.
+
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+. "${srcdir=.}/tests/init.sh"; path_prepend_ ./src
+print_ver_ tail
+
+NL='
+'
+
+touch "$NL" normal || framework_failure_
+
+tail -n1 "$NL" normal >out || fail=1
+
+cat<<\EOF>exp || framework_failure_
+==> ''$'\n' <==
+
+==> normal <==
+EOF
+
+compare exp out || fail=1
+
+Exit $fail