Explain Linux file permissions. What does chmod 644 mean, and what would 755 be for?
What they are really testing: A screen-out: do you actually understand the user/group/other model and octal notation, or just memorize numbers. They want you to decode the bits and connect them to a real use (a file vs a directory or script).
A real interview question
Explain Linux file permissions. What does chmod 644 mean, and what would 755 be for?
What most people say
drag me
“644 gives read and write, 755 gives full permissions.”
It does not break the number into owner/group/other or the 4/2/1 bits, and "full permissions" is wrong (755 is not 777). It shows pattern-memory, not the model.
The follow-ups they ask next
What is the difference between chmod and chown?
chmod changes the permission bits; chown changes the owner (and group, with chown user:group). Different problems: who can do what vs who it belongs to.
What does the setuid bit do and why is it risky?
setuid makes an executable run as its owner (often root) regardless of who launches it, useful for tools like passwd, but a classic privilege-escalation risk if set on the wrong binary.
What the interviewer is listening for
- Breaks numbers into owner/group/other
- Knows the 4/2/1 values
- Knows execute means traverse on a directory
What sinks the answer
- Calls 755 "full permissions"
- Can't decode an octal value
- Confuses chmod and chown
If you genuinely do not know
Say this instead of freezing. Reasoning out loud from what you do know beats silence every single time, and a good interviewer is listening for exactly that.
“Permissions cover [owner, group, other], each with [read 4, write 2, execute 1] summed. So 644 is [owner rw, group r, other r, a data file] and 755 is [owner rwx, group/other rx, a script or directory]. Note [execute on a directory means traverse].”
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