Day 5 - Week 4 Review: Permissions on One Page

📌 Key Takeaways

• Permissions are four digits: [special][owner][group][other]. If the first digit is 0, it is omitted.
• r=4 w=2 x=1, SetUID=4 SetGID=2 Sticky=1 — both lines use the same 4·2·1.
• umask is subtraction. It is subtracted from 666 for files and 777 for directories. The default 022 → 644 / 755.
• chown is root only, while chmod can also be done by the owner.
• df is the file system, du is the path. umount has no n.

Permissions on One Page

   chmod  4  7  5  5   file
          │  │  │  │
          │  │  │  └── other   r-x
          │  │  └───── group   r-x
          │  └──────── user    rwx
          └─────────── special SetUID
                       4 SetUID · 2 SetGID · 1 Sticky
 
   -rws r-x r-x
    │└┬┘ └┬┘ └┬┘
    │ │   │   └── other
    │ │   └────── group
    │ └────────── owner (the s in the x slot = SetUID)
    └──────────── file type

   umask calculation
 
     file        666  -  umask
     directory   777  -  umask
 
     022  →  file 644 · directory 755   (default)
     077  →  file 600 · directory 700   (strict)
     000  →  file 666 · directory 777   (a file is not 777!)

Octal Conversion Table — Memorize This

Contrasting Easily Confused Pairs

Commonly Missed Points

1. With umask 000, a file is 777 → No. The file base value is 666, so it is 666. It is the directory that becomes 777.
2. umask is addition → It is subtraction. The larger the value, the narrower the permissions.
3. x on a directory is execute → It is enter. Without x, cd doesn't work.
4. A read-only file can't be deleted → It can be deleted. Deletion is decided by the directory's w, not the file. What prevents this is the Sticky bit.
5. If you're the owner, you can do everything → No. Only the single user slot applies. If the owner slot is narrow, the owner can't read.
6. You can hand your own file to someone else → chown is possible for root only.
7. unmount → It is umount. No n.
8. Running fsck while mounted → There's a risk of damage. Always do it after unmounting.
9. Special permission order as 1·2·4 → It is 4 (SetUID) · 2 (SetGID) · 1 (Sticky). Memorize it in the same order as rwx.
10. With umask 033, a file is 633 → It is 644. umask just switches off the permissions that are on, so there is nothing to subtract for the x that the file never had.
11. SetUID runs with root privileges → Precisely, it is the privileges of the file's owner. When the owner is root, it becomes root privileges.
12. With the Sticky bit set, you can't read others' files → What it blocks is only deletion and renaming. Read and write are decided by the file's own permissions.
13. df /var/log is the size of that directory → It is the capacity of the entire file system that directory belongs to. The directory size is du -sh.
14. chmod -r for subdirectories → Recursion is uppercase -R. The same for chown and chgrp.

Calculations to Work Out by Hand

In the exam room, you have to convert on the spot with no calculator. Let's follow one of each type.

Type ① Symbols ↔ Numbers

   rwxr-x--x   →  (4+2+1)(4+0+1)(0+0+1)  →  751
   rw-rw----   →  (4+2+0)(4+2+0)(0+0+0)  →  660
   640         →  6=rw-  4=r--  0=---    →  rw-r-----
   711         →  7=rwx  1=--x  1=--x    →  rwx--x--x

• Quick check: odd digits have x. 6, 4, 2, and 0 can't execute; 7, 5, 3, and 1 can.

Type ② Results of the Symbolic Method

Start from rw-r--r-- (644) and apply them in order.

• + and - change only what's written, and = replaces all three slots of that target. g=rw erases the permissions the group had and rewrites it as rw-.

Type ③ umask

• If you fix an order, you won't go wrong — find the directory value first (777 − umask), and removing the x from it gives the file value.
• There are questions that ask in reverse, too. "A new directory is created as 750. What is the umask?" → 777 − 750 = 027.

Type ④ Reading Special Permissions

• An s in the first three slots is SetUID, an s in the middle three slots is SetGID, and a t at the very end is Sticky.

What Can This Person Do

The key to permission questions is sorting out "who gets which slot." alice and bob are in the dev group, and carol is not.

   drwxrwx--T  2 root  dev   /project
   -rw-r-----  1 alice dev   /project/plan.txt

• The order of reasoning is always the same — ① which slot applies (owner → group → other, just one) ② is the target a file or a directory ③ if it's deletion, it is the directory's permissions and the Sticky bit, not the file's.

Collected Commands and Options

One-Line Summaries of Directories

Values That Appear on the Exam Exactly

Next week covers the program that receives and interprets commands — the shell. Environment variables, redirection, and aliases come up.

📖 Glossary

• Special permission slot : The first digit of the four permission digits. If 0, it is omitted and written as three digits.
• umask : The value to trim from the permissions of new files and directories. The base is 666 for files and 777 for directories.
• SetUID / SetGID / Sticky bit : Special permissions with the values 4, 2, and 1 respectively.
• Mount point : The directory where a file system is attached.
• FHS : The Filesystem Hierarchy Standard that fixes directory names and roles.
• inode exhaustion : A state where capacity remains but no more files can be created. Check with df -i.