readlink: support multiple command line arguments

This allows efficient processing of multiple files,
while also increasing compatibility with BSD's readlink(1).
We also add the -z, --zero option to delimit output items
with the NUL character which disambiguates output in the
presence of '\n' characters.

* src/readlink.c (usage): Add the --zero description,
and also adjust the description of --no-newline accordingly.
(main): Handle the -z option and iterate over multiple arguments.
Also as in commit v8.15-24-g9d46b25 we use fputs() and putchar()
rather than printf() for performance reasons.
* doc/coreutils.texi (readlink invocation): Document the
new --zero option, adjust the --no-newline description, and
tweak the general info to indicate multiple files are supported.
* tests/readlink/multi.sh: A new test for the new functionality.
* tests/local.mk: Reference the new test.
* man/readlink.x: Adjust the summary and also reference realpath.
* NEWS: Mention the improvement.
* THANKS.in: Suggested by Aaron Davies.

1 files changed, 11 insertions, 4 deletions

diff --git a/doc/coreutils.texi b/doc/coreutils.texi
index 5f8fad774..0646f8242 100644
--- a/doc/coreutils.texi
+++ b/doc/coreutils.texi
@@ -9737,20 +9737,20 @@ Set the default SELinux security context to be used for created files.
 
 @item Readlink mode
 
-@command{readlink} outputs the value of the given symbolic link.
+@command{readlink} outputs the value of the given symbolic links.
 If @command{readlink} is invoked with an argument other than the name
 of a symbolic link, it produces no output and exits with a nonzero exit code.
 
 @item Canonicalize mode
 
-@command{readlink} outputs the absolute name of the given file which contains
+@command{readlink} outputs the absolute name of the given files which contain
 no @file{.}, @file{..} components nor any repeated separators
 (@file{/}) or symbolic links.
 
 @end table
 
 @example
-readlink [@var{option}] @var{file}
+readlink [@var{option}]@dots{} @var{file}@dots{}
 @end example
 
 By default, @command{readlink} operates in readlink mode.
@@ -9789,7 +9789,8 @@ as a directory.
 @itemx --no-newline
 @opindex -n
 @opindex --no-newline
-Do not output the trailing newline.
+Do not print the output delimiter, when a single @var{file} is specified.
+Print a warning if specified along with multiple @var{file}s.
 
 @item -s
 @itemx -q
@@ -9807,6 +9808,12 @@ Suppress most error messages.
 @opindex --verbose
 Report error messages.
 
+@item -z
+@itemx --zero
+@opindex -z
+@opindex --zero
+Separate output items with @sc{nul} characters.
+
 @end table
 
 The @command{readlink} utility first appeared in OpenBSD 2.1.