is the default permissions mode on Debian.

chmod u+x myfile
This command means to add (+) execute (x) permissions for the user (u) who
owns the file. Use ls -l to see the effects.

chmod go-r myfile
Here you've subtracted (-) read permission (r) from the group (g) owning
the file and from everyone else (others, o). Again, use ls -l to verify
the effects.

chmod ugo=rx myfile
Here you've set (=) user, group, and other permissions to read and
execute. This sets permissions to exactly what you've specified and unsets
any other permissions. So all rx should be set, and all w should be unset.
Now, no one can write to the file.

chmod a-x myfile
a is a shortcut for ugo, or ``all.'' So all the x permissions should now
be unset.

rm myfile
With this command, you're removing the file, but without write
permissions. rm will ask if you're sure by displaying the following
message:

rm: remove `myfile', overriding mode 0444?
You should respond by typing y and pressing Enter. This is a feature of
rm, not a fact of permissions. Permission to delete a file comes from the
directory permissions, and you have write permission in the directory.