cp: change --attributes-only to not truncate existing files

* src/copy.c (copy_reg): Don't truncate an existing file,
to support copying attributes between existing files.
The original use case only considered creating new files,
and it would be a very unusual use case to be relying
on the truncating behavior.
* doc/coreutils.texi (cp invocation): Mention the non
truncating behavior.
* tests/cp/attr-existing: A new test to ensure O_TRUNC skipped.
* tests/Makefile.am: Reference the new test.
* NEWS: Mention the change in behavior.

1 files changed, 29 insertions, 0 deletions

diff --git a/tests/cp/attr-existing b/tests/cp/attr-existing
new file mode 100755
index 000000000..9cf0ffcb3
--- /dev/null
+++ b/tests/cp/attr-existing
@@ -0,0 +1,29 @@
+#!/bin/sh
+# Make sure cp --attributes-only doesn't truncate existing data
+
+# Copyright 2012 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 <http://www.gnu.org/licenses/>.
+
+. "${srcdir=.}/init.sh"; path_prepend_ ../src
+print_ver_ cp
+
+printf '1' > file1
+printf '2' > file2
+printf '2' > file2.exp
+
+cp --attributes-only file1 file2 || fail=1
+cmp file2 file2.exp || fail=1
+
+Exit $fail