Outcome

  • Understand how rwx maps to octal digits.
  • Set, fix, and audit permissions quickly using:
    • chmod
    • chown
    • setfacl
  • Remember what SUID / SGID / sticky and ACL do without Googling.

Permissions Basics

OctalSymbolicMeaning
0---No rights
1--xExecute only
2-w-Write only
3-wxWrite + Execute
4r--Read only
5r-xRead + Execute
6rw-Read + Write
7rwxRead + Write + Execute

Mnemonic: 421 $\rightarrow$ rwx

Just add them to get each digit.

Example:

chmod 640 file $\rightarrow$ rw- r-- ---

(owner can read/write, group can read, others have no permissions).

Cheat-sheet for common tasks

Task	 Command
Give owner execute	chmod u+x script.sh
Remove “others” write	chmod o-w *.md
Exact mode (octal)	chmod 755 /usr/local/bin/tool
Change owner & group	sudo chown ivan:devs project
Recursive group write	chmod -R g+w shared/
Default mask for new files	umask 002
  • UID / GID – User / Group ID numbers.
  • ACL – Access Control List, extra per-user rules beyond rwx.

Special bits

Bit	Octal flag	What it does
SUID	4	Run program as file owner (often root)
SGID	2	Run as file’s group or make new files inherit the dir’s group
Sticky	1	On dirs: only owner may delete own files (e.g. /tmp)
  • Combine by prepending the flag digit: chmod 4755 /bin/foo sets SUID + 755.

ACLs in one breath

  • Why? rwx covers only owner / group / others; ACL adds per-user or per-group rules.
  • Add rule: setfacl -m u:alice:rw file
  • View rules: getfacl file
  • Works on ext4, XFS, APFS
  • Windows NTFS uses ACLs by default (icacls).

Handy commands to remember

  • cat /etc/os-release – check distro info
  • ls -l – view permissions (symbolic)
  • stat -c %a file – view octal mode
  • find . -type f ! -perm 644 – spot odd perms
  • systemctl list-units –type=service – list active services

Octal memory drill

Write this on scratch paper, then quiz yourself:

r (4)   w (2)   x (1)

7 = rwx   6 = rw-   5 = r-x   4 = r--
3 = -wx   2 = -w-   1 = --x   0 = ---
  • Pick random numbers; translate to symbols & back.
  • Aim for < 5 s per conversion.
  • Practice: chmod –verbose 640 dummyfile and read the output.

Todos

  • Add /proc & /sys permission quirks.
  • One-pager on symbolic vs octal chmod.
  • Compare Linux ACL vs Windows NTFS ACL.