shred: refactor to clean up code and reduce syscalls

src/uu/shred/Cargo.toml

@@ -18,6 +18,7 @@ path = "src/shred.rs"
 clap = { workspace=true }
 rand = { workspace=true }
 uucore = { workspace=true }
+libc = { workspace=true }
 
 [[bin]]
 name = "shred"

src/uu/shred/shred.md

@@ -0,0 +1,47 @@
+# shred
+
+<!-- spell-checker:ignore writeback -->
+
+```
+shred [OPTION]... FILE...
+```
+
+Overwrite the specified FILE(s) repeatedly, in order to make it harder for even
+very expensive hardware probing to recover the data.
+
+## After help
+
+Delete `FILE(s)` if `--remove` (`-u`) is specified. The default is not to remove
+the files because it is common to operate on device files like `/dev/hda`, and
+those files usually should not be removed.
+
+CAUTION: Note that shred relies on a very important assumption: that the file
+system overwrites data in place. This is the traditional way to do things, but
+many modern file system designs do not satisfy this assumption. The following
+are examples of file systems on which shred is not effective, or is not
+guaranteed to be effective in all file system modes:
+
+ * log-structured or journal file systems, such as those supplied with
+ AIX and Solaris (and JFS, ReiserFS, XFS, Ext3, etc.)
+
+ * file systems that write redundant data and carry on even if some writes
+ fail, such as RAID-based file systems
+
+ * file systems that make snapshots, such as Network Appliance's NFS server
+
+ * file systems that cache in temporary locations, such as NFS
+ version 3 clients
+
+ * compressed file systems
+
+In the case of ext3 file systems, the above disclaimer applies (and shred is
+thus of limited effectiveness) only in `data=journal` mode, which journals file
+data in addition to just metadata. In both the `data=ordered` (default) and
+`data=writeback` modes, shred works as usual. Ext3 journal modes can be changed
+by adding the `data=something` option to the mount options for a particular
+file system in the `/etc/fstab` file, as documented in the mount man page (`man
+mount`).
+
+In addition, file system backups and remote mirrors may contain copies of
+the file that cannot be removed, and that will allow a shredded file to be
+recovered later.