chmod.md

Change File Permissions

This lesson covers using chmod to set file and directory permissions. Remember to use chmod carefully, as modifying permissions can have significant security implications.

chmod is a command in Unix-based operating systems that allows users to change the permissions of a file or directory. The name "chmod" stands for "change mode," and it's a powerful tool for securely managing file permissions.

There are three basic types of permissions for a file or directory:

1. Read (r) permission: Allows a user to view the contents of a file or directory.

2. Write (w) permission: Allows a user to modify the contents of a file or directory.

3. Execute (x) permission: Allows a user to run a file or access a directory's contents.

Each permission can be assigned to three groups of users:

1. Owner (u): The user who owns the file or directory.

2. Group (g): A group of users who have access to the file or directory.

3. Others (o): Any user who is not the owner or a member of the group.

The chmod command uses a numerical value to represent the permissions for each group of users:

• Read permission is represented by 4

• Write permission is represented by 2

• Execute permission is represented by 1

• No permission is represented by 0

To set permissions for a file or directory using chmod, you need to specify the numerical value for each group of users and the file or directory you want to modify.

Here's an example of how to use chmod:

1. Open a terminal window and navigate to the directory that contains the file you want to modify.

2. Use the ls -l command to view the current permissions of the file.

3. Determine the permissions you want to set for the file or directory.

4. Use the chmod command to set the permissions. For example, to give the owner read, write, and execute permissions, the group read permission, and others no permission, you would enter:

chmod 750 filename.

5. Again, use the ls -l command to verify that the permissions have been set correctly.

Summary Permissions Reference Tables

Octal	Binary	Permission	Description
0	000	---	No permissions
1	001	--x	Execute only
2	010	-w-	Write only
3	011	-wx	Write and execute
4	100	r--	Read only
5	101	r-x	Read and execute
6	110	rw-	Read and write
7	111	rwx	Read, write, and execute

Command	Permissions	Meaning
chmod 777	rwxrwxrwx	Full access for user, group, and others
chmod 755	rwxr-xr-x	User full; group and others can read & exec
chmod 700	rwx------	Full user access; no access for others
chmod 644	rw-r--r--	User can read/write; others read only
chmod 600	rw-------	User read/write; no one else has access

Permission	User (u)	Group (g)	Others (o)
Read (r)	4	4	4
Write (w)	2	2	2
Execute (x)	1	1	1

Permission	User (u)	Group (g)	Others (o)
Read	r (4)	r (4)	r (4)
Write	w (2)	w (2)	w (2)
Execute	x (1)	x (1)	x (1)

Octal	Symbolic	Meaning
700	rwx------	Full access for user only
755	rwxr-xr-x	User full, others read & execute
644	rw-r--r--	User read/write, others read
600	rw-------	User read/write only
777	rwxrwxrwx	Full access for everyone

Octal	Symbolic	Meaning
0	---	No permissions
1	--x	Execute
2	-w-	Write
3	-wx	Write and execute
4	r--	Read
5	r-x	Read and execute
6	rw-	Read and write
7	rwx	Read, write, execute

Symbol	Effect
+	Adds permission
-	Removes permission
=	Sets exact permission

Command	Meaning
chmod +x file	Add execute permission for all (u/g/o)
chmod u+x file	Add execute for user only
chmod g-w file	Remove write from group
chmod o=r file	Set read-only for others
chmod a+x script.sh	Add execute to all for a script
chmod 755 file	User: rwx, Group/Others: r-x
chmod 644 file	User: rw-, Group/Others: r--
chmod -R 755 dir/	Recursively set rwxr-xr-x for all files/dirs

Scripts need chmod +x script.sh before running with ./script.sh

Assignment Exercises

This assignment covers using chmod to set file and directory permissions. Remember to use chmod carefully, as modifying permissions can have significant security implications. Good luck with your chmod practice!

1. Open a terminal window and navigate to a directory containing one or more files.

2. Use the ls -l command to view the current permissions of the files.

3. Determine the permissions you want to set for one of the files.

4. Use the chmod command to set the permissions for that file. For example, to give the owner read and write permissions, the group read permission, and others no permission, you would enter: chmod 640 filename.

5. Again, Use the ls -l command to verify that the permissions have been set correctly.

6. Create a new file in the same directory and set its permissions to give the owner read, write, and execute permissions, the group read and execute permissions, and others no permission. Use the chmod command to set the permissions.

7. Use the ls -l command to verify that the new file's permissions have been set correctly.

8. Modify the permissions of a directory in the same directory to give the owner, group, and others read and execute permissions. Use the chmod command to set the permissions.

9. Use the ls -l command to verify that the directory's permissions have been set correctly.