Question 1

How do octal permissions map to read, write, and execute?

Accepted Answer

Each permission class (owner, group, other) is one octal digit built by adding read = 4, write = 2, and execute = 1. So rwx = 7, rw- = 6, r-x = 5, and r-- = 4. The common 755 means owner has rwx (7) while group and other have r-x (5), and 644 means owner rw- with group and other read-only.

Question 2

What is the difference between chmod 755 and 644?

Accepted Answer

755 (rwxr-xr-x) is typical for directories and executable scripts — the owner can read, write, and execute while everyone else can read and execute. 644 (rw-r--r--) is typical for regular files — the owner can read and write while everyone else can only read. Files that do not need to be run should not have the execute bit.

Question 3

What are setuid, setgid, and the sticky bit?

Accepted Answer

They are special permission bits set with a fourth leading octal digit. setuid (4000) makes an executable run with the file owner's privileges; setgid (2000) runs it with the group's privileges, or makes new files in a directory inherit that directory's group; the sticky bit (1000) on a shared directory such as /tmp restricts file deletion to each file's owner.

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