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authorPaul Eggert <eggert@cs.ucla.edu>2017-02-11 23:12:31 -0800
committerPaul Eggert <eggert@cs.ucla.edu>2017-02-11 23:14:02 -0800
commit376967889ed7ed561e46ff6d88a66779db62737a (patch)
tree7ca32f07b738fc2f9902c1643bb01dedb7d1066b /doc
parent2f69dba5df8caaf9eda658c1808b1379e9949f22 (diff)
downloadcoreutils-376967889ed7ed561e46ff6d88a66779db62737a.tar.xz
ln: replace destination links more atomically
If the file B already exists, commands like 'ln -f A B' and 'cp -fl A B' no longer remove B before creating the new link. Instead, they arrange for the new link to replace B atomically. This should fix a race condition reported by Mike Crowe (Bug#25680). * NEWS, doc/coreutils.texi (cp invocation, ln invocation): Document this. * bootstrap.conf (gnulib_modules): Add symlinkat. * src/copy.c, src/ln.c: Include force-link.h. * src/copy.c (same_file_ok): It's also OK to remove a destination symlink when creating symbolic links, or when the source and destination are on the same file system and when creating hard links. * src/copy.c (create_hard_link, copy_internal): * src/ln.c (do_link): Rewrite using force_linkat and force_symlinkat, to close a window where the destination temporarily does not exist. * src/cp.c (main): Do not set x.unlink_dest_before_opening merely because we are in link-creation mode. * src/force-link.c, src/force-link.h: New files. * src/local.mk (copy_sources, src_ln_SOURCES): Add them. * tests/cp/same-file.sh: Adjust test case to match fixed behavior.
Diffstat (limited to 'doc')
-rw-r--r--doc/coreutils.texi18
1 files changed, 11 insertions, 7 deletions
diff --git a/doc/coreutils.texi b/doc/coreutils.texi
index 3eac96b7d..2dbfccecc 100644
--- a/doc/coreutils.texi
+++ b/doc/coreutils.texi
@@ -8125,9 +8125,11 @@ Equivalent to @option{--no-dereference --preserve=links}.
When copying without this option and an existing destination file cannot
be opened for writing, the copy fails. However, with @option{--force},
when a destination file cannot be opened, @command{cp} then removes it and
-tries to open it again. Contrast this behavior with that enabled by
-@option{--link} and @option{--symbolic-link}, whereby the destination file
-is never opened but rather is removed unconditionally. Also see the
+tries to open it again. When this option is combined with
+@option{--link} (@option{-l}) or @option{--symbolic-link}
+(@option{-s}), the destination link is replaced, and unless
+@option{--backup} (@option{-b}) is also given there is no brief
+moment when the destination does not exist. Also see the
description of @option{--remove-destination}.
This option is independent of the @option{--interactive} or
@@ -9825,11 +9827,13 @@ directory, using the @var{target}s' names.
@end itemize
-Normally @command{ln} does not remove existing files. Use the
-@option{--force} (@option{-f}) option to remove them unconditionally,
-the @option{--interactive} (@option{-i}) option to remove them
+Normally @command{ln} does not replace existing files. Use the
+@option{--force} (@option{-f}) option to replace them unconditionally,
+the @option{--interactive} (@option{-i}) option to replace them
conditionally, and the @option{--backup} (@option{-b}) option to
-rename them.
+rename them. Unless the @option{--backup} (@option{-b}) option is
+used there is no brief moment when the destination does not exist;
+this is an extension to POSIX.
@cindex hard link, defined
@cindex inode, and hard links