chmod

Understanding File Permissions

Before understanding chmod, it’s essential to grasp the fundamental concept of file permissions in Linux. Each file and directory possesses three sets of permissions:

• Owner: The user who created the file or directory.
• Group: A group of users who share access privileges.
• Others: All other users on the system.

Each set has three permissions:

• Read (r): Allows viewing the file’s contents (for files) or listing its contents (for directories).
• Write (w): Allows modifying the file’s contents (for files) or adding/deleting files within the directory (for directories).
• Execute (x): Allows running the file (for executable files) or accessing the directory (for directories).

These permissions are represented numerically (4 for read, 2 for write, 1 for execute) or symbolically (r, w, x).

Using chmod with Numerical Notation

The numerical method expresses permissions as a three-digit octal number. Each digit corresponds to the permissions for owner, group, and others respectively.

Example 1: Setting permissions to 755

The command chmod 755 myfile.txt sets the permissions as follows:

• 7 (Owner): 4 + 2 + 1 = Read, Write, and Execute permissions for the owner.
• 5 (Group): 4 + 1 = Read and Execute permissions for the group.
• 5 (Others): 4 + 1 = Read and Execute permissions for others.

Example 2: Restricting access

To make a file readable only by the owner:

chmod 400 sensitive_data.txt

This sets permissions to 4 (read) for the owner, 0 (no permissions) for the group, and 0 (no permissions) for others.

Using chmod with Symbolic Notation

The symbolic method is more user-friendly and easier to remember. It uses the u, g, o, and a symbols to represent owner, group, others, and all respectively, followed by the +, -, or = operators to add, remove, or set permissions.

Example 3: Adding execute permission for the group

chmod g+x my_script.sh

This adds execute permission (x) for the group (g).

Example 4: Removing write permission for others

chmod o-w myfile.txt

This removes write permission (w) for others (o).

Example 5: Setting permissions for all users

chmod a=rx my_directory

This sets read (r) and execute (x) permissions for all (a).

Working with Directories

The chmod command works identically for directories. Execute permission on a directory allows access to its contents.

Example 6: Making a directory accessible to everyone

chmod a+rx my_public_directory

This adds read and execute permissions for everyone on my_public_directory.

Advanced Scenarios and Troubleshooting

Remember to always double-check your chmod commands before execution. Incorrectly setting permissions can lead to data inaccessibility. Using the ls -l command will help you verify the permissions of a file after executing chmod.

Further exploration of umask can assist in setting default permissions for newly created files and directories. Understanding these concepts is fundamental to securing your Linux system effectively.