Question 1

What does `chmod 755` actually do?

Accepted Answer

Owner gets rwx (read + write + execute = 4+2+1 = 7), group and others get r-x (read + execute = 4+1 = 5). Standard for executables and directories — the owner can modify, everyone else can read and traverse. It's the default you want for a script you just `chmod +x`-ed.

Question 2

What's wrong with `chmod 777`?

Accepted Answer

777 means 'anyone on the system can read, write, and execute this file'. On a shared system or a server, that's an open invitation — any compromised process can modify your file. Almost every time `chmod 777` 'fixes' something, the real fix is a narrower permission (usually 644 or 755) and maybe a `chown`.

Question 3

When do I use 644 vs 755?

Accepted Answer

644 for regular files (owner can edit, others read-only). 755 for directories (you need execute/x on a directory to `cd` into it and list contents) and for scripts you want to run. A common mistake: `chmod 644` on a directory and then wondering why `ls` says 'permission denied'.

Question 4

What are setuid, setgid, and sticky bit?

Accepted Answer

Those are the fourth-digit permissions you sometimes see as `4755` or `1777`. Setuid (`4xxx`) makes a binary run with the owner's privileges — used by `sudo`. Sticky bit (`1xxx`) on a directory means only the owner can delete their own files — used by `/tmp`. Rarely needed in day-to-day use; use them cautiously when you do.

Question 5

Does this work for Windows permissions too?

Accepted Answer

No. Windows uses ACLs, not Unix-style rwx bits. For Windows, you're looking at `icacls` or PowerShell's `Get-Acl`/`Set-Acl`. This tool is Unix/Linux/macOS only.

chmod Calculator

What is chmod Calculator?

Related tools