Question 1

What does chmod stand for?

Accepted Answer

chmod stands for 'change mode'. It is a Unix/Linux command used to change the access permissions of files and directories.

Question 2

What do the three octal digits represent?

Accepted Answer

The three digits represent owner (user), group, and others — in that order. Each digit is a sum of read (4), write (2), and execute (1) bits.

Question 3

When should I use 755 vs 644?

Accepted Answer

Use 755 for directories and executable scripts (owner full access, everyone else read+execute). Use 644 for regular files (owner read+write, everyone else read-only).

Question 4

What are setuid, setgid, and the sticky bit?

Accepted Answer

setuid (4xxx) runs an executable as the file's owner. setgid (2xxx) runs it as the file's group, or makes new files in a directory inherit the group. The sticky bit (1xxx) restricts deletion in a directory to the file's owner only — commonly used on /tmp.

Question 5

Why is 777 considered insecure?

Accepted Answer

chmod 777 gives read, write, and execute permission to everyone on the system. Any user can modify or execute the file, which is a security risk in multi-user environments or on web servers.

chmod Calculator

chmod Calculator