# CHMOD

The chmod command is used to change file and directory permissions in Linux and Unix-based systems. Permissions can be specified using either numeric (octal) or symbolic (letter) notation.

## Numeric (Octal) Notation

Each permission is represented by a number:
```
4 = Read (r)
2 = Write (w)
1 = Execute (x)
```

The values are added together for each user category:
```
Owner
Group
Others
```

Example: `chmod 755 script.sh`

This results in:
```
Owner: rwx (7 = 4+2+1)
Group: r-x (5 = 4+1)
Others: r-x (5 = 4+1)
```

## Symbolic (Letter) Notation

Permissions can also be modified using letters:
```
u = User (owner)
g = Group
o = Others
a = All users
```

Operators:
```
+ = Add permission
- = Remove permission
= = Set exact permission
```

Examples:
```
chmod u+x script.sh      # Add execute permission for the owner
chmod g-w file.txt       # Remove write permission from the group
chmod a=r file.txt       # Set read-only permission for everyone
```

Both notations achieve the same result. Numeric notation is commonly used for setting complete permission sets, while symbolic notation is useful for modifying specific permissions.

## Special Bits
| Bit | Octal | Letter | on file | on directory |
| - | - | - | - | - |
| Setuid | 4 | u+s | program runs with uid of file | --- |
| Setgid | 2 | g+s | program runs with gid of file | directory passes group of directory to new files/directories beneath |
| Sticky | 1 | +t | --- | the files within the directory are only deletable and renamable from owner of directory or file |

example:
```
3770  Setgid + Sticky-Bit + rwx for user, rwx for group and none for other
```