Add du cheat sheet and one exhaustive example

sheets/du

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+# du
+# Estimate file space usage
+
+# With 'root' privileges, use du(1), sort(1), and head(1) to display a list of
+# the top 20 space-consuming files in whichever storage medium '/' is mounted.
+#
+# Here, du(1) is using the `-x` flag to keep to the one filesystem, which is
+# important for getting accurate results on the filesystem on which you
+# might, for example, be needing to free space.
+#
+# In order to sort the human-readable file sizes, sort(1) is using the `-h`
+# flag, the `-k` flag to specify the column to sort (first), and its using
+# the `-r` flag to reverse the sorting, so we see the highest size first.
+#
+# To then show the top-20 lines, we use head(1) and specify the number of lines
+# via the `-n` flag. The default number of lines displayed by head(1) and
+# tail(1) is 10.
+#
+# Root privileges are gained for this task by using sudo(8) on bash(1) in order
+# to have a new root-owned BASH session, which then executes the commands
+# proceeding the `-c` flag.
+sudo bash -c 'du -xh / | sort -rhk 1 | head -n 20'