File and Directory Permissions

• The lab focuses on understanding file and directory permissions to ensure security in computing environments.
• The lab teaches about read, write, and execute permissions and their relevance to users and groups.
• Students will practice changing permissions using command-line tools.
• This will help them manage access and protect files in system administration.

Lab Learning Outcomes

• Identify different types of file and directory permissions (read, write, execute).
• Understand the significance of user, group, and other classifications.
• Use command-line tools to change file and directory permissions.
• Safeguard files and directories through proper permission settings.

Lab Instructions

• The lab instructor provides an overview of file and directory permissions.
• Labs highlight their critical role in system security.
• The instructor will demonstrate command-line tools for managing permissions.
• They will provide hands-on exercises for changing permissions effectively.

File and Directory Permissions

• File permissions are core to the Linux security model.

• They determine who can access files and directories and how.

• Linux employs security measures to protect users' files from unauthorized access.

• The "ls" command by default, only shows file names without detail.

• Users can add options starting with '-' to view more details.

• The "ls -l" (long listing) command provides a detailed view with each file on a separate line.

• File type is indicated by a character: '-' for regular file, 'd' for directory, 'l' for symbolic link.

• Permission settings are displayed as a string like "rw-r--r--".

• User owner indicates the owner of the file with “root” as example.

• Group owner indicates the group that owns the file with “root” as example.

• Size of the file is indicated in bytes, such as 46 bytes.

• Date and time signifies when the file was last modified.

• File_name or Directory_name provides corresponding object name (example).

Permission Groups in Linux

• Think of file permissions as three sets of three "rwx" characters corresponding to read, write, execute operations.
• Owners, permissions apply to file/directory owners.
• Groups permissions can be assigned to user groups, affecting only those users.
• All Users permissions apply to all system users. Using them presents the highest security risk, so assignment should be cautious.

Permission Class References

• u (user): Applies only to the file owner, not affecting other users.
• g (group): Applies only to the assigned group, not affecting other users.
• o (others): Applies to all other users on the system and needs monitoring
• a (all): Applies to all three categories: owner, group, others.

File Permissions in Linux

• Read, write, and execute are the three file permissions.
• ‘r’ reads the file’s contents.
• ‘w’ writes, or modifies, the file’s contents.
• ‘x’ executes the file, this permission is for programs.

Operators for Linux Permissions

• + add permissions
• - remove permissions
• = set permissions to the specified values.

Security Permissions Examples

• "rw-r-xr--"
• "rw-": Owner can read and write, but not execute.
• "r-x": Group members can read and execute, but not write.
• "r--": Other users can only read.
• Users in Linux world can read but cannot modify or execute the files' contents.

Changing Permissions in Linux

• Permissions are also known as modes.
• The command chmod is used to change permissions.
• The format for chmod commands is chmod mode file where "mode" can be specified in two ways: symbolic or octal.
• The symbolic mode format is chmod user_category operator permission.

Symbolic Mode Components for chmod

• Chmod the change mode command.
• ugoa user (u), group (g), other (o), all (a).
• += + to add permissions, - to subtract, = to set explicitly.
• rwx read (r), write (w), execute (x).

Changing Permissions Examples

• To add write permission for the members of a file's group, the steps are:

• Create file name sales.data: touch sales.data

• Display information about file sales.data: ls -l sales.data

• Change permissions to add write permission for group: chmod g+w sales.data

• Display new information about file sales.data: ls -l sales.data

• Use chmod g+wx sales.data

• The display new information includes using ls -l sales.data

• To set permissions on different user categories simultaneously: chmod ug+wx sales.data

• The display new information includes using ls -l sales.data

• The user add rwx for user and add x for group

• To set different permissions for different user categories, separate specifications with a comma.

• The display new information includes using ls -l sales.data

• Example: chmod u=rwx,g+x sales.data will specify rwx for user while adding x for group.

• To make a file readable by everyone, use chmod a=r file.

• The equal sign (=) sets permissions exactly as specified.

• If no permissions are specified the equal sign removes them.

• The display new information includes using ls -l sales.data

• Example: To remove all permissions: chmod u=rwx,g=rx,o= sales.data

Numeric Based Permissions

• Octal mode can set file/directory permissions (alternative to symbolic mode).
• Each permission type is a bit, either off (0) or on (1).
• Order matters, always rwx.
• If r, w, and x are all off, binary representation is 000. If all on, it is 111. To show read and execute and omitting write shows the binary as 101.

Binary and Decimal Value Conversions

• 0 0 0 Binary value for off
• 1 1 1 Binary value for on
• 0 0 0 Base 10 (decimal) value for off
• 4 2 1 Base 10 (decimal) value for on
• The shortcut to convert binary to base 10 is read equals 4, write equals 2, and execute equals 1.

Octal Permissions

• 0 (000 ---) No permissions
• 1 (001 --x) Execute only
• 2 (010 -w-) Write only
• 3 (011 -wx) Write and execute (2+1)
• 4 (100 r--) Read only
• 5 (101 r-x) Read and execute (4+1)
• 6 (110 rw-) Read and write (4+2)
• 7 (111 rwx) Read, write, and execute (4+2+1)

Commonly Used Permissions

• 700 (-rwx------) ensures only the owner can read, edit, and execute the file.
• 755 (-rwxr-xr-x) allows anyone to execute but only the owner can edit the file.
• 664 (-rw-rw-r--) allows a group to modify the file while letting others read it.
• 660 (-rw-rw----) allows a group to modify the file and not letting others read it.
• 644 (-rw-r--r--) allows everyone to read the file, but owner can edit it.

Permissions Changes

• Display the permissions when changing by using the ls -l command.
• Specify the numeric code command using: chmod 664 EX