du: add -t SIZE, --threshold=SIZE option

* src/du.c (opt_threshold): Add variable to hold the value of
the --threshold option specified by the user.
(long_options): Add a required_argument entry for the new
--threshold option.
(usage): Add --threshold option.
(process_file): Elide printing the entry if its size does not
meet the value specified by the --threshold option.
(main): In the argument parsing loop, add a case for the new
-t option. Convert the given argument by permitting the
well-known suffixes for megabyte, gigabytes, etc.
Handle the special case "-0": give an error as this value is
not permitted.
* doc/coreutils.texi (du invocation): Add documentation for the
above new option.
* tests/du/threshold.sh: Add new test to exercise the new option.
* tests/local.mk (all_tests): Mention the above test.

Co-authored-by: Bernhard Voelker <mail@bernhard-voelker.de>

1 files changed, 35 insertions, 0 deletions

diff --git a/doc/coreutils.texi b/doc/coreutils.texi
index 45a4b3d16..8d7eff764 100644
--- a/doc/coreutils.texi
+++ b/doc/coreutils.texi
@@ -11022,6 +11022,41 @@ With @option{--separate-dirs}, the size reported for a directory name,
 @var{d}, is merely the @code{stat.st_size}-derived size of the directory
 entry, @var{d}.
 
+@item -t @var{size}
+@item --threshold=@var{size}
+@opindex -t
+@opindex --threshold
+Exclude entries based on a given @var{size} (@pxref{Block size}).
+
+If @var{size} is positive, then @command{du} will only print entries with a size
+greater than or equal to that.
+
+If @var{size} is negative, then @command{du} will only print entries with a size
+smaller than or equal to that.
+
+Although GNU @command{find} can be used to find files of a certain size,
+@command{du}'s @option{--threshold} option can be used to also filter
+directories based on a given size.
+
+Please note that the @option{--threshold} option can be combined with the
+@option{--apparent-size} option, and in this case would elide entries based on
+its apparent size.
+
+Here's how you would use @option{--threshold} to find directories with a size
+greater than or equal to 200 megabytes:
+
+@example
+du --threshold=200MB
+@end example
+
+Here's how you would use @option{--threshold} to find directories and files -
+note the @option{-a} - with an apparent size smaller than or equal to 500 bytes:
+
+@example
+du -a -t -500 --apparent-size
+@end example
+
+
 @item --time
 @opindex --time
 @cindex last modified dates, displaying in @command{du}