Linux File Permissions

Read (4): Allows looking inside the file.
Write (2): Allows modifying file content.
Execute (1): Allows running a file as a program.

Every file has an inode which acts as its record, containing timestamps, owner, and group information.

Linux permissions are divided into three sets: Owner, Group, and Others. The system checks these in order; the first match wins.

Read	Write	Execute
4	2	1

Chmod Numbers: Permissions are represented by numeric values (4, 2, 1) that add up to a digit (e.g., 7 is read + write + execute).
Common modes include 755 for programs and 644 for standard files.
Directory Permissions: Directories function as tables of contents.
- Read lets you list contents (ls).
- Execute is the permission to enter or traverse a directory.
- Write permits adding or deleting files within that directory. This explains why you can delete a file you cannot read: you are modifying the directory's list, not the file itself.
Troubleshooting Script Execution (If a script fails to execute)
check the execute bit
directory path permissions
the presence of a shebang (e.g., #!/bin/bash), and
verify the interpreter path.

Special Bits

Setuid: Allows a program to run with the privileges of the file owner (e.g., passwd).
Setgid: Useful for shared directories; new files inherit the group of the directory.
Sticky Bit: Prevents users from deleting files owned by others in a shared directory like /tmp.

Common Patterns