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A Practical Guide to Linux® Commands, Editors, and Shell Programming, Third Edition by Mark G. Sobell

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chmod: Changes Access Permissions

The Linux file access permission scheme lets you give other users access to the files you want to share yet keep your private files confidential. You can allow other users to read from and write to a file (handy if you are one of several people working on a joint project). You can allow others only to read from a file (perhaps a project specification you are proposing). Or you can allow others only to write to a file (similar to an inbox or mailbox, where you want others to be able to send you mail but do not want them to read your mail). Similarly you can protect entire directories from being scanned (covered shortly).

Security: A user with root privileges can access any file on the system

There is an exception ...

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