Permissions
Linux permissions use an owner, group, and others model to control access to files and directories.
Overview
- Permission bits define read, write, and execute access
- Files and directories interpret those bits differently
chmod, chown, and special bits modify access rules
Reference
Permission types
| Symbol |
Meaning |
r |
Read |
w |
Write |
x |
Execute |
Permission scope
| Scope |
Meaning |
u |
Owner |
g |
Group |
o |
Others |
File vs directory semantics
| Object |
r |
w |
x |
| File |
Read contents |
Modify contents |
Execute file |
| Directory |
List entries |
Create or remove entries |
Traverse directory |
- Directory traversal requires
x, even if r is set
x on a directory does not mean you can execute files inside it- Changing directory contents requires
w on the directory itself
Representation
ls -l /etc/passwd
-rwxrw-r-- 1 root root 1641 May 4 23:42 /etc/passwd
| Segment |
Meaning |
- |
File type |
rwx |
Owner permissions |
rw- |
Group permissions |
r-- |
Others permissions |
1 |
Hard link count |
root root |
Owner and group |
1641 |
File size |
May 4 23:42 |
Last modified |
Octal notation
| Value |
Permission |
4 |
Read |
2 |
Write |
1 |
Execute |
| Example |
Meaning |
7 |
rwx |
6 |
rw- |
5 |
r-x |
4 |
r-- |
Modifying permissions
chmod a+r shell
chmod 754 shell
chown <user>:<group> <file-or-directory>
chmod adds or removes permissions with symbolic or numeric modeschown changes the owner and optionally the group
Special bits
- SUID and SGID run a program with the file owner’s or group’s privileges
- The sticky bit prevents users from deleting files they do not own in shared directories
- In
ls -l, s replaces x for SUID or SGID, and t marks the sticky bit
Mental Model
- Ownership sets the baseline
- Groups extend access
- Permission bits gate operations
- Special bits change how execution and deletion behave