This document provides an overview of files, users, and permissions in Linux. It discusses that Linux is Unix-like and everything is treated as a file. Permissions include read, write and execute and are based on the file's user, group and others. It also covers users and groups, and how permissions can be changed using chmod, chown and chgrp. The root user has full privileges to do anything on the system. Scripts require execute permission to run.

1. Linux Tips: Files, Users & Permissions
Fabio Emilio Costa
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2. Some History
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Linux == Unix-Like
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Quacks like duck, walks like duck, but no duck
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Unix was created on 1960s
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Multi-user
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How avoid users to access forbidden files
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Permissions
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Groups
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Everything is treated as a file on Unix (directories, devices)

3. Permissions
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Read (4) – cat, cp ...
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Write (2) – gedit, vi, emacs, rm..
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eXecute (1) – run as programs (scripts)
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Numbers were normally used
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On directories:
–
Read: ls...
–
Write: touch, rm...
–
Execute: cd...

4. Users
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Permissions are given on a file based on:
–
–
Group: people who are from the same group the owner;
–
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User: who created the file (normally);
Others: nuff said...
ls -l shows all the normal permissions

5. Reading permissions on ls -l
File type
● - – common file
● d – directory
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b – block device (disks)
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c – character devices (terms)
● l – symbolic link
Owner's permissions
Group's permissions
Other's permissions
● read
● write
● execute
● - – unset permission
-rw-------

6. Users & Groups
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All permissions are based on users
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Normally indicated by either a name and/or a number
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On appliance, jharvard is user 1000
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Can be seen on /etc/passwd file
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Users normally are parts of groups
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Groups can be used for accounting, security, administration;
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A user can be part of one or more groups;
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jharvard is part of wheel group (can use sudo)
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Can be seen on /etc/group file

7. Manipulating permissions
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Changing Permissions: chmod (Change Mode);
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Changing the Owner of a file: chown (Change Owner);
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Changing the Group of a file: chgrp (Change Group);
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Just root can chown and chgrp;
–
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On older Linux and Unix, owner could chown and
chgrp, but that was a security risk (SetUID);
Only root and owner can chmod;

8. Changing permissions with chmod
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By numbers: sum the permissions numbers. Each number
represents one into User, Group, Others (UGO)
–
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Character indication: placing the needed permissions for a kind
after a = symbol;
–
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chmod u=rw,go=w bar
Set/Unset: using the + symbol to set a permission and a –
symbol to unset a permission (easier method);
–
●
chmod 644 foo
chmod a+r,u+w baz
Special group: All

9. The root
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Special user
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Can do ANYTHING with a system...
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… including break it
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Has a special ID of 0
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Many commands can be done just by root...
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… or users that can use sudo, from wheel group …
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… and jharvard is from wheel group

10. "With great power comes great
responsibility"
(PARKER, Ben)

11. Want to see Linux being broken?
Running rm -rf / on Linux
http://www.youtube.com/watch?v=D4fzInly
Don't try this at home!!!!!

12. Scripts
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On Windows, files are opened based on extension;
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.exe, .com, .bat – programs/scripts;
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On Unix, needs eXecution permission to run as a script;
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Any file with eXecution permission will be executed (if
possible);

13. THIS IS CS50