Permissions: rwx, Octal & Special Bits

What is it?

Each file has read/write/execute bits for user, group and other — shown as rwxr-xr-- and set numerically (chmod 754). Directories use x to mean 'may enter'.

Why it matters

Permissions are the everyday access control; over-permissive files (world-writable scripts, 777) are a top cause of both breakage and privilege escalation.

Where you see it

`ls -l` shows the bits; `chmod 640 file` sets them; `chmod u+x script.sh` adds execute for the owner.

What normal looks like

Config files 640 (owner rw, group r), private keys 600, scripts 750 — least privilege that still works.

What suspicious looks like

A private key at 644 (readable by all), a world-writable cron script (a root-execution backdoor), or 777 'to make it work'.

How analysts investigate

Read the rwx triplets against who needs access, and treat any world-writable executable or over-readable secret as a finding to fix, not to keep.

Common beginner mistakes

Using 777 to 'fix' access instead of the least bits that work.
Forgetting a directory needs execute (x) to be entered, not just read.

Read the triplets

  -rwxr-x---   =  750   (u=rwx, g=r-x, o=---)
     u  g  o
  4=r 2=w 1=x → 7=rwx 5=r-x 0=---
  private key  → 600      cron script → NOT world-writable
  world-writable exe = anyone can change what root runs

rwx bits per user/group/other, set as octal; over-permissive bits are a security finding.

Quick check

What octal gives owner rwx, group r-x, others nothing?

A quick self-check — it doesn't affect your XP or progress.