chmod, chown

ChatGPT

2025-01-24

In Unix-like systems, files and directories have permissions that define who can read, write, or execute them. These permissions are represented using a 3-digit octal system, where each digit corresponds to different user categories: owner, group, and others.

Each permission type is represented by a number:

Permission	Decimal	Binary
Read (r)	4	100
Write (w)	2	010
Execute (x)	1	001

The values for each user category are added together to form a 3-digit octal number. Each of the three digits represents the permissions for a specific category:

Owner’s permissions (first digit)
Group’s permissions (second digit)
Others’ permissions (third digit)

For example:

chmod 755 filename
- Owner (7) = Read (4) + Write (2) + Execute (1) = rwx
- Group (5) = Read (4) + Execute (1) = r-x
- Others (5) = Read (4) + Execute (1) = r-x

`chmod`

The chmod (change mode) command is used to modify the permissions of a file or directory. Permissions are expressed in either symbolic or numeric form, and the command allows you to set or change who can read, write, or execute a file.

Specifying the User (Owner, Group, Others)

In addition to numeric notation, chmod allows you to specify which user (owner, group, or others) you want to modify permissions for using symbolic notation.

u = user (owner)
g = group
o = others
a = all (owner, group, others)

To modify specific user permissions, you can use the following symbolic operators:

+ = add permission
- = remove permission
= = set exact permission

Examples of Symbolic Usage

Command	Description
`chmod +x filename`	Add execute permission for everyone
`chmod u+x filename`	Add execute permission for the user (owner)
`chmod g-w filename`	Remove write permission for the group
`chmod o=r filename`	Set others to read-only permission
`chmod u+x,g-w filename`	Add execute for the user and remove write for the group
`chmod a+r filename`	Add read permission for everyone (user, group, others)

`chown`

The chown command is used to change the owner and/or group of a file or directory. It doesn’t directly change the permissions but modifies who owns the file and which group it belongs to.

Command	Description
`chown user filename`	Change the owner of the file to the specified user
`chown user:group filename`	Change both the owner and group of the file
`chown :group filename`	Change the group of the file, keeping the owner unchanged
`chown -R user:group directory/`	Recursively change ownership of all files in a directory