added cp, mv, and dd to the filesystem quick-ref

filesystem.txt

@@ -38,6 +38,39 @@ Moving Around
 | popd        | Pop the top of the directory stack, cd'ing to the new top      |
 
 
+Moving, Copying, and Extracting Pieces of Files
+-----------------------------------------------
+
+| *Command*        | *Result*                                               |
+| ---------        | --------                                               |
+| cp *a* *b*       | Create a copy of file *a* named *b*                    |
+| mv *a* *b*       | Rename file *a* so that it is now *b*                  |
+| dd if=*a* of=*b* | Dump the contents of input file *a* to output file *b* |
+
+For all of these the files may be in any directory, so to move a
+file `foo` up a directory, you could run
+
+    mv foo ../foo
+
+or just
+
+    mv foo ../
+
+`dd` is an extremely powerful tool, and `if` and `of` are only the
+beginning of its options. Either can be omitted, and use STDIN or
+STDOUT as the default.  Here's another simple example:
+
+    dd if=/dev/zero of=foo bs=1024 count=5
+
+This will create a file `foo` with the first 5kB of `/dev/zero`,
+which will provide you with as many NULL bytes as you request.  Give
+this a try, and then run
+
+    xxd foo
+
+See the manpage for `dd` for all options. It's well worth you time
+to learn more about this command!
+
 File Permissions
 ----------------