Understanding file permissions and modifying them using chmod

PLease do not publeish this until come changes are made

replace Linux by gnu/linux
make sure that different points are in different lines not in paragraphs

Linux is basically a multi-user system. But nowadays there has been an
increase in the usage of Linux as an OS for single user home computers
as well. But since it was originally meant to be a multi-user OS, a lot
of things that would be necessary in a multi-user system are built into
the Linux core. One such thing is file / directory permissions.
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