Cheatsheet & Examples: chmod
chmod is a command-line utility used to change the access permissions of file system objects such as files and directories. It allows you to control who can read, write, and execute those objects.
Changing Permissions Using Octal Notation
Example Usage:
chmod 755 myfile.txt
What it does:
Sets the permissions of myfile.txt. The owner gets read, write, and execute permissions (7), the group gets read and execute permissions (5), and others get read and execute permissions (5).
Command-line Arguments Explained:
755: The octal representation of the desired permissions. The first digit (7) represents the owner's permissions, the second digit (5) represents the group's permissions, and the third digit (5) represents the others' permissions. Each digit is calculated as the sum of its permission bits:- 4 (read)
- 2 (write)
- 1 (execute)
- 7: (4+2+1) Read, write, and execute
- 5: (4+0+1) Read and execute
myfile.txt: The name of the file whose permissions are to be changed.
Changing Permissions Using Symbolic Notation
Example Usage:
chmod u+rwx,g+rx,o+rx myfile.txt
What it does:
Grants the owner read, write, and execute permissions, the group read and execute permissions, and others read and execute permissions to myfile.txt.
Command-line Arguments Explained:
u: Represents the user/owner.g: Represents the group.o: Represents others (all users not in the owner or group).+: Adds the specified permissions.-: Removes the specified permissions.=: Sets the specified permissions, removing all others.r: Read permission.w: Write permission.x: Execute permission.rwx: Combines read, write, and execute permissions.myfile.txt: The name of the file whose permissions are to be changed.
Removing Permissions
Example Usage:
chmod u-w myfile.txt
What it does:
Removes write permission from the owner of myfile.txt.
Command-line Arguments Explained:
u: Represents the user/owner.-: Removes the specified permissions.w: Write permission.myfile.txt: The name of the file whose permissions are to be changed.
Granting Execute Permission to a Directory
Example Usage:
chmod +x mydirectory
What it does:
Grants execute permission to all users on the directory mydirectory. This is necessary to allow users to cd into the directory and access its contents.
Command-line Arguments Explained:
+: Adds the specified permissions.x: Execute permission.mydirectory: The name of the directory whose permissions are to be changed.
Setting Permissions Recursively
Example Usage:
chmod -R 755 mydirectory
What it does:
Applies the permissions 755 to mydirectory and all files and subdirectories within it, recursively.
Command-line Arguments Explained:
-R: (Recursive) Applies the changes to the directory and all of its contents.755: The octal representation of the desired permissions.mydirectory: The name of the directory whose permissions are to be changed.
Changing the Owner of a File
Example Usage:
chown user:group myfile.txt
What it does:
Changes the owner and group ownership of myfile.txt. Note: This uses the chown command, which is often used in conjunction with chmod.
Command-line Arguments Explained:
user: The username of the new owner.:group: The name of the group to which the file will belong.myfile.txt: The name of the file.
Preserving the Existing Permissions and Adding
Example Usage:
chmod +a "group:mygroup allow read,write,execute" myfile.txt
What it does:
Adds Access Control List (ACL) permissions. Note: requires acl support on the filesystem.
Command-line Arguments Explained:
+a: Adds a new Access Control List (ACL) entry."group:mygroup allow read,write,execute": Specifies that the groupmygrouphas read, write, and execute permissions.myfile.txt: The name of the file whose permissions are to be changed.
