Understanding chmod 777 vs. 755 Permissions: Why 777 Isn't Always utable?

The confusion regarding why chmod 777 might not always make a file utable whereas chmod 755 does can often stem from a misunderstanding of how Unix-like systems manage file permissions. This article aims to clarify this confusion by breaking down the concepts involved.

Understanding Permission Numbers

File permissions in Unix-like operating systems are defined using octal numbers, which can range from 0 to 777. These numbers represent a combination of three types of permissions: read (r), write (w), and execute (x).

Octal Representation

The permissions numeric values are as follows:

4 - Read (r) 2 - Write (w) 1 - Execute (x)

For instance, a permission value of 7 (4 2 1) would mean the file is readable, writable, and executable by the owner, while a value of 5 (4 1) would mean the file is readable and executable.

Permission Breakdown

Each digit in the octal number represents permissions for different user categories, specifically:

Owner (first digit) Group (second digit) Others (third digit)

Examples

A breakdown of the permission values for chmod 777 and chmod 755 is as follows:

chmod 777 means: Owner: Read (4) Write (2) Execute (1) 7 Group: Read (4) Write (2) Execute (1) 7 Others: Read (4) Write (2) Execute (1) 7

This means that chmod 777 would allow all users read, write, and execute permissions on the file. In contrast:

chmod 755 means: Owner: Read (4) Write (2) Execute (1) 7 Group: Read (4) Execute (1) 5 Others: Read (4) Execute (1) 5

Here, the owner has full permissions, but the group and others can only read and execute the file.

Why chmod 777 Might Not Be utable

While chmod 777 does provide execute permissions to all users, it does not necessarily mean the file is executable. There are several reasons for this:

File Type: For a file to be executable, it must be a script or binary designed to run. Having execute permissions alone is insufficient if the file content is not executable. Filesystem Settings: Some filesystems have additional mount options that can restrict execution regardless of the file permissions. Security Policies: Security policies like SELinux or AppArmor, or user-specific restrictions, can prevent the execution of files.

Executing a file requires the file to have a valid executable format and the system to recognize it as such. Even if you grant execute permissions, a file that is not a valid executable will not be utable.

Conclusion

In summary, while chmod 777 does provide execute permissions to all users, it does not guarantee that a file will be executable if it does not meet other criteria. On the other hand, chmod 755 ensures that the owner can read, write, and execute a file, while denying these privileges to the group and others, which is a common permission setting for scripts and binaries intended for execution.