Skip to content

03_Linux_File_Permissions_Notes.md

Latest commit

 

History

History
211 lines (147 loc) · 6.3 KB

03_Linux_File_Permissions_Notes.md

File metadata and controls

211 lines (147 loc) · 6.3 KB
Raw

Linux — File Permission & Access Modes

Short & clear revision notes · 7 topics

01 · What are File Permissions?

Every file in Linux has three levels of ownership, and each level gets its own set of permissions:

Level Who it applies to
Owner The user who created/owns the file
Group All users who belong to the file's group
Others Everyone else on the system

🔐 File permissions are the first line of defence in Linux security. They control exactly who can do what with every file.

02 · Reading Permissions — ls -l

Running ls -l shows permissions in the first column. Example:

-rwxr-xr-- 1 amrood users 1024 Nov 2 00:10 myfile

Breaking down the permission string

- rwx r-x r--
│ │   │   │
│ │   │   └── Others:  read only
│ │   └─────── Group:   read + execute
│ └─────────── Owner:   read + write + execute
└───────────── File type: - = file, d = directory
Position Characters Refers to
1 - or d or l File type
2–4 rwx Owner permissions
5–7 r-x Group permissions
8–10 r-- Others permissions

Each position is either its letter (r, w, x) if allowed, or - if not.

03 · The Three Permissions — r, w, x

For files

Permission Symbol What it allows
Read r View the contents of the file
Write w Modify or delete the file's content
Execute x Run the file as a program

For directories

Permission Symbol What it allows
Read r View the list of filenames inside the directory
Write w Add or delete files inside the directory
Execute x Traverse the directory — enter it and access its contents

💡 For directories, execute is really a traverse permission. You need x on /bin just to run ls or cd.

04 · Changing Permissions — chmod

chmod (change mode) is used to change permissions. There are two ways to use it.

Method 1 — Symbolic Mode

Use letters and operators to add, remove, or set permissions.

Who Symbol Operators Permission
owner u + add r read
group g - remove w write
others o = set exactly x execute
all three a

Examples:

Command What it does
chmod o+wx testfile Add write + execute for others
chmod u-x testfile Remove execute from owner
chmod g=rx testfile Set group to read + execute only
chmod o+wx,u-x,g=rx testfile Combine multiple changes in one line

Method 2 — Absolute (Octal) Mode

Each permission has a numeric value. Add them up for each group (owner/group/others).

Number Permission Symbol
0 No permission ---
1 Execute --x
2 Write -w-
3 Write + Execute (2+1) -wx
4 Read r--
5 Read + Execute (4+1) r-x
6 Read + Write (4+2) rw-
7 All permissions (4+2+1) rwx

Examples:

Command Result Meaning
chmod 755 testfile -rwxr-xr-x Owner: all · Group: r+x · Others: r+x
chmod 743 testfile -rwxr---wx Owner: all · Group: r · Others: w+x
chmod 043 testfile ----r---wx Owner: none · Group: r · Others: w+x

💡 Quick cheat: 755 = standard for executables, 644 = standard for regular files, 777 = full access for everyone (use carefully).

05 · Changing Owner & Group

Command Syntax What it does
chown chown user filelist Change the owner of a file
chgrp chgrp group filelist Change the group of a file

Examples:

chown amrood testfile      # Change owner to amrood
chgrp special testfile     # Change group to 'special'

⚠️ Only the root (superuser) can change ownership of any file. Normal users can only change ownership of files they own.

06 · SUID & SGID Bits

Sometimes a program needs to run with elevated permissions temporarily — for example, passwd needs to write to /etc/shadow even though regular users can't touch that file.

This is solved with SUID and SGID bits:

Bit Full name What it does
SUID Set User ID Program runs with the owner's permissions, not the caller's
SGID Set Group ID Program runs with the group owner's permissions

How to spot them

The s letter appears in place of x in the execute position:

$ ls -l /usr/bin/passwd
-r-sr-xr-x 1 root bin 19031 Feb 7 13:47 /usr/bin/passwd
      ^
      s here = SUID is set (runs as root, whoever calls it)

💡 A lowercase s means the execute bit is also set. A capital S means execute is NOT set (SUID is set but the file isn't executable).

Setting SUID/SGID

chmod ug+s dirname
# Result: drwsr-sr-x

07 · Sticky Bit

The sticky bit on a directory adds an extra layer of protection — even if others have write access to the directory, they can only delete files they own.

A file inside a sticky directory can only be removed by:

  • The owner of the sticky directory
  • The owner of the file being removed
  • The superuser (root)

💡 The classic example is /tmp — everyone can write there, but no one can delete someone else's files.

Quick Reference Cheatsheet

Permission string breakdown:
  - rwx r-x r--
  │ │   │   └── Others (8-10)
  │ │   └─────── Group  (5-7)
  │ └─────────── Owner  (2-4)
  └───────────── File type

Octal values:
  r = 4,  w = 2,  x = 1
  7 = rwx,  6 = rw-,  5 = r-x,  4 = r--

Common permission sets:
  chmod 755   →  rwxr-xr-x  (executables)
  chmod 644   →  rw-r--r--  (regular files)
  chmod 700   →  rwx------  (private to owner only)
  chmod 777   →  rwxrwxrwx  (full open — use with care)

Linux File Permissions Notes — for revision use