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→INVESTIGATION 2: FILE PERMISSIONS
:* Explain the purpose of '''file permissions'''
:* Explain how permissions work differently for '''directories''' as opposed to for '''regular files'''
:* Change file '''permissions''' with the '''chmod''' command (both ''symbolic'' and ''absolute'' methods)
:* Use the '''umask''' command to automatically assign permissions for '''newly created directories ''' and '''regular files'''<br><br>
===Tutorial Reference Material===
<table align="right"><tr valign="top"><td>[[Image:binary-number1.png|thumb|right|220px|A series of '''binary numbers''' form a '''byte''' to represent '''numbers'''.<br>(Image licensed under [https://creativecommons.org/licenses/by-sa/3.0/ cc])]]</td><td>[[Image:binary-number-2.png|thumb|right|200px|'''Bytes''' can be used to also represent '''characters'''. It is job of a program to know if bytes are used to represent '''numbers''' or '''characters'''. Learning to convert numbering systems<br>(like '''Hexadecimal to Binary''') can be used to know how a character is represented in '''binary'''. <br>(Image licensed under [https://creativecommons.org/licenses/by-sa/3.0/ cc])]]</td></tr></table>
<i>''Data ... is any sequence of one or more symbols given meaning by specific act(s) of interpretation. Digital data is data that is represented using the binary number system of ones (1) and zeros (0)...''</i><br>
Reference: https://en.wikipedia.org/wiki/Data_(computing)<br><br>
''Examples of Understanding IT Professionals that Use Data Representation:''
* ''Network Specialists'': Building Large Networks via Sub-netting
* ''C ProgrammingProgrammers:'' Sending information over networks, files* ''Web DevelopmentDevelopers:'' Setting color codes for webpage background or text * ''Allowing or Limiting Unix / Linux File AccessSystem Administrators:'' Setting ''permissions'' for files and directories
<br><br>
''Decimal Numbers''
The '''decimal''' numbering system consists of digits consisting of numbers ranging from '''0''' to '''9'''. The numbering system is based on '''sums of the power of 10''' which provides a framework for mathematic calculations.
The fact that '''humans''' started counting on their fingers and thumbs most likely lead to the development of this numbering system.
''Binary Numbers''
The '''binary''' numbering system consists of digits containing the numbers ranging from '''0''' or to '''1'''. The numbering system is based on '''sums of the power of 2'''.
Digital computers have circuits which representing data in terms of voltage levels. Multiple circuits are used to represent data (in the form of ''binary'' numbers).
''Octal / Hexadecimal Numbers''
<table align="right"><tr valign="top"><td>[[Image:octal-number-2.png|thumb|right|200px|The '''octal''' numbering system.<br>(Image licensed under [https://creativecommons.org/licenses/by-sa/3.0/ cc])]]</td><td>[[Image:hex-number-2.png|thumb|right|420px|The '''hexadecimal''' numbering system.<br>(Image licensed under [https://creativecommons.org/licenses/by-sa/3.0/ cc])]]</td></tr></table>
The '''octal''' and '''hexadecimal''' numbering systems consist of digits containing numbers ranging from '''0 to 7''' and ranging from '''0 to F''' respectively. For ''hexadecimal'' numbers, values for '''10 to 15''' are represented by the characters '''A to F''' respectively. The '''octal''' and '''hexadecimal''' numbering system are based on sums of the power of '''8''' and '''16''' respectively.
Since '''1 octal digit represents 3 binary digits''' and '''1 hexadecimal digit represents 4 binary digits''', octal and hexadecimal numbers are used as '''short-cuts''' to represent binary number.
'''NOTE:''' In this course, we cannot create groups or assign users to groups in the '''Matrix''' server.<br>Instead, you will learn how to those tasks when or if you take a Unix/Linux administration course.<br>On the other hand, you can change which '''user''', '''same group members''' or '''other group members'''<br>can access or NOT access a directory or regular file.<br><br>
[[Image:directory-permissions.png|thumb|right|350px|Permissions of a '''directory''' that contain subdirectories and regular files.]]
File Permissions consist of '''two-layers''':<br><br>'''First''', the permissions of a '''directory''' that contains regular files, and '''second''', permissions of the ''subdirectories and/or regular files'' within a that directory.
A symbol ''dash'' "'''-'''" indicates that the permission has '''NOT''' been granted.
[[Image:file-permissions.png|thumb|right|350px|Permissions of a '''regular file''' contained within a directory.]]
The permissions of '''newly-created''' directories and regular files are automatically assigned<br>via a '''user mask''' (we will discuss this shortly). In order to change permissions for directories and regular files, >you would use the '''chmod''' command.
'''Perform the Following Steps:'''
# Let's convert the following ''8-bit'' binary number '''10111110''' to a '''decimal ''' number.<br><br><span style="color:blue;">'''NOTE:''' It is important to learn and '''memorize''' the '''correct techniques''' to perform the<br>proper numbering conversion method (i.e. view '''method 1''' above (drawing the L's).</span><br><br>
# Write the manual conversion on a sheet of paper.<br><br>
# Use a '''calculator''' to check your work. In MS Windows, you can set the calculator to Programming mode<br>by making the selection to '''binary''', enter the binary number '''10111110''' and view the decimal equivalent.<br><br>Did you get the correct answer? If not, retry the method and check to see what you did wrong.<br><br>
'''Perform the Following Steps:'''
# '''Login''' to your matrix account and issue a command to '''confirm''' you are located in your '''home''' directory.<br><br>
# Issue a single Linux command to create the following directory structure displayed in the diagram to the right.<br><br>
# Make certain that your current directory is '''your home directory'''.<br><br>'''NOTE:''' You will now run a shell script to confirm that you properly created that directory structure<br>in your ''Matrix'' account.<br><br>
# Issue the following Linux command: <span style="color:blue;font-weight:bold;font-family:courier;">chmod 750 -R ~/documents</span><br><br>
# Again, use the '''ls''' command to confirm the permissions for the '''~/documents''', '''~/document/memos''' , '''~/documents/reports''', and '''~/documents/contracts''' directories.<br><br>
# Issue the following Linux command: <span style="color:blue;font-weight:bold;font-family:courier;">ls -lR ~/documents</span><br>What do you noticed happened to the permissions for the regular files contained in those directories. <br>Did those regular file permissions change?<br><br>We want to now change those regular file permissions to the following settings: <span style="font-weight:bold;font-family:courier;">r w - r - - - - -</span><br><br>
# Issue the following Linux commands: <br><span style="color:blue;font-weight:bold;font-family:courier;">chmod 640 ~/documents/memos/memo*.txt<br>chmod 640 ~/documents/reports/report*.txt<br>chmod 640 ~/dcouments/contracts/contract*.txt</span><br><br>
# Issue the '''ls -lR''' command for the '''~/documents''' directory to confirm that those regular file permission have changed.<br><br>Let's get some practice setting permissions to allow users to make editing changes to regular files.<br><br>
# Issue the following Linux command to view permissions for your '''home''' directory: <span style="color:blue;font-weight:bold;font-family:courier;">ls -ld ~</span><br><br>What does execute permissions mean for same group members and other group members in terms of your '''home''' directory?<br><br>
# Issue the following Linux command to create a new subdirectory: <span style="color:blue;font-weight:bold;font-family:courier;">mkdir ~/shared</span><br><br>
# Issue the following Linux command: <span style="color:blue;font-weight:bold;font-family:courier;">ls -l ~/shared</span><br><br>What are the permissions for this newly-created directory? <br>Can other users access the directory pathname '''~youruserid/shared''' ?<br><br>
# Issue the following Linux command (without an argument): <span style="color:blue;font-weight:bold;font-family:courier;">umask</span><br><br>'''NOTE:''' You should see a '''four-digit octal''' number. Drop the leading zero on the left to obtain the '''default umask value'''.<br><br>
# Perform a '''mathematical calculation ''' by taking the octal number '''777''' and subtracting the default umask value you determined in the previous step.<br>What is the result?<br><br>
# Convert that octal number result to a '''binary''' number. What does that represent as newly created directory permissions?<br>Does that correspond to the permissions for the newly created '''~/shared''' directory?<br><br>
# Repeat '''steps #27''' and '''#28''' to see how a umask setting of '''077''' would affect permissions of newly-created directories.<br><br>
# Issue the following Linux command: <span style="color:blue;font-weight:bold;font-family:courier;">umask 077</span><br><br>
# Issue the following Linux command (without arguments): <span style="color:blue;font-weight:bold;font-family:courier;">umask</span><br><br>NOTE: You should notice the value '''0077'''. By dropping the leading zero to the left, that would provide the default '''umask value of 077'''.<br><br>
# Issue the following Linux command: <span style="color:blue;font-weight:bold;font-family:courier;">mkdir ~/shared2</span><br><br>
