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Unix Access Permission Bits: Read, Write, Execute

Permission bits are a crucial aspect of access control primitives on a Unix system, as they determine the accessibility of files and directories for users and groups. Unix employs a set of three permission bits for each file or directory, representing the access rights granted to the owner, group, and other users. These permission bits play a vital role in maintaining the security, integrity, and privacy of data on Unix systems. The following sections describe the ways in which permission bits are necessary for files using access control primitives on a Unix system:
  1. Read (r): The read permission bit determines whether a user or group can read the contents of a file or list the contents of a directory. Granting the read permission is necessary to allow users to view and open files, while restricting read access can protect sensitive data from unauthorized access.
  2. Write (w): The write permission bit controls the ability to modify a file's content or create, rename, or delete files within a directory. Write permissions are essential for enabling users to create, edit, and delete files, but should be carefully managed to prevent unauthorized modification or deletion of critical data.
  3. Execute (x): The execute permission bit governs the ability to execute a file as a program or script or access the contents of a directory. For executable files, this permission is necessary to allow users to run programs or scripts. In the case of directories, the execute permission allows users to traverse the directory structure and access subdirectories, even if they cannot list the directory contents.

Permission bits are typically represented using a combination of the letters 'r', 'w', and 'x' for each category of user (owner, group, and others). For example, a file with the permission string '-rwxr-xr--' indicates that the owner has read, write, and execute permissions, the group has read and execute permissions, and others have only read permission.
The importance of permission bits in access control primitives on a Unix system can be summarized as follows:
  1. Security: Permission bits are vital for maintaining the security of a Unix system by restricting unauthorized access to sensitive data and preventing accidental or malicious modification of critical files.
  2. Data Integrity: By controlling write access, permission bits help maintain data integrity by preventing unauthorized users from modifying files, which could result in data corruption or loss.
  3. Privacy: Read permissions enable administrators to protect the privacy of users' personal data by restricting access to specific files and directories.
  4. System Stability: Execute permissions allow administrators to control which users can run executable files or scripts, ensuring that only authorized users can execute programs and preventing potential system instability caused by unauthorized execution.

In conclusion, permission bits are an essential aspect of access control primitives on a Unix system, serving to maintain security, data integrity, privacy, and system stability by granting or restricting access to files and directories based on user and group permissions.

Meaning of access permission bits

The permission bits have the following significance for files:
  1. Read (r) permission means that the file contents may be read.
  2. Write (w) permission means that the file contents may be altered (so that, for example, the file may be appended to, overwritten, or truncated). Note that deleting a file involves write permission on the directory, not the file itself.
  3. Execute (x) permission means that the file may be loaded and executed (assuming that it is a program file).

The permission bits have the following significance for directories:
  1. Read (r) permission means that the directory contents may be read from the disk. Therefore, the filenames in the directory can be listed.
  2. Write (w) permission means that the directory may be altered. In particular, you can delete files from the directory or rename files in the directory.
  3. Execute (x) permission means you can make this directory your current directory. More important, execute permission means that you can search the directory. You cannot access a directory at all unless you have execute permission for every directory higher in the path of a given directory. Without execute permission in the given directory, you can list only the filenames in the directory.

Octal numbers and meanings for a user of a file


Octal Number Meaning
0 0 0 0 No permission
1 0 0 1 Execute permission, no read and write permissions
2 0 1 0 Write permission, no read and execute permissions
3 0 1 1 Write and execute permissions, no read permission
4 1 0 0 Read permission, no write and execute permissions
5 1 0 1 Read and execute permissions, no write permission
6 1 1 0 Read and write permissions, no execute permission
7 1 1 1 Read, write and execute permissions