Difference Between chmod and chown

The chmod and chown commands are used to control access to files in UNIX and Linux systems.

The chmod command stands for "change mode", and allows changing permissions of files and folders, also known as "modes" in UNIX. The chown command stands for "change owner", and allows changing the owner of a given file or folder, which can be a user and a group. That's the difference between them in a nutshell.

They are interrelated in so far as changing ownership of a file changes who the set permissions apply to. The new owner inherits the permissions.

Let's take a quick look at the basic usage of these commands.

chmod

The chmod command can be used in a couple of different ways, with permissions (or modes) set by numbers or by letters. Permissions can be given to a user who owns the file (u = user), group of said user (g = group), everyone else (o = others) or all users (a). And the basic permissions that can be given include read (r), write (w), and execute (x). There are also X, s, and t, but they're less commonly used.

When using numbers you can use a numeric value such as 644 to set permissions. The position of the value represents to whom is the permission given, and the actual value represents which (or how much) permissions are given as a sum total of each permission's unique value.

First position (in the above example 6) refers to the user. Second refers to the group of the user, and the third refers to all others.

Numeric values for permissions are:

4 = read 2 = write 1 = execute

So a value of 4 will only give read rights, but a value of 6 will give read and write rights because it is a sum of 4 and 2. 5 will give only read ane execute rights, and 7 will give all rights. Do this calculation for each numerical position and you'll end up with the desired value. So in the example of 644 we're giving the user who owns the file the permission to read and write (but not execute), the group of that user the permission to read only, and others the right to read only as well.

To set this mode with chmod on a file called important.txt we would simply run this command:

chmod 644 important.txt

Note that making a file executable, if it were a script or a program, amounts to simply giving someone or everyone a permission to execute. If this was an important.sh bash script we could allow the owner to execute, and others to read with the 744 mode, or everyone to execute with 755.

chmod 755 important.sh

Now, we can also use letters to accomplish the same thing, and we've already mentioned the relevant letters above. This is probably easier to remember than using numbers. For example, to accomplish the 644 permissions above we would run this:

chmod u+rw,go+r important.txt

So we're saying file owner user gets read and write permissions, group and others get to read.

The second example, with the important.sh file being made executable we could just run this:

chmod u+rwx,go+rx important.sh

If important.sh already had permissions set to 644 we can add everyone execute rights by simply running:

chmod +x important.sh

Not specifying the letter for anyone is treated as if we said "a", for all.

Finally, if we're setting permissions to a folder we need to specify the -R option (standing for "recursive"):

chmod -R 644 important-files/

chown

Basic usage of chown is pretty straightforward. You just need to remember that first comes the user (owner), and then the group, delimited by a colon.

This command will set the user "daniel", from the group of "admins" as owners of the directory "important-files":

chown -R daniel:admins important-files

Just like with chmod, the -R is there when it's a directory.

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Chmod Command Examples in Unix / Linux

Chmod (change mode) is one of the most frequently used commands in unix or linux operating system. The chmod command is used to change the file or directory access permissions. To know about the access permissions of a file or directory, use the ls -l command as shown below:

$ ls -l sample.sh
-rwx-rw-r-- 1 matt deploy 94 Oct  4 03:12 sample.sh

Here in the above example: Use matt has the read, write and execute permissions on the file. Group deploy has read and write permissions. Others have only the read permission.
File and Directory permissions:

There are three different permissions. They are: 

◈ Read (4): Permitted to read the contents of the file. In case of directory, you can view all the files and sub-directories in that directory.
◈ Write (2): Permitted to write to the file. In case of directory, you can create files and sub-directories.
◈ Execute (1): Execute the file as a program/shell script. In case of directory, You can enter into that directory.

Here in the above, the numbers in the brackets represents the numeric values for the corresponding permissions. If you want to have a combination of permissions add the required numbers. For example, for read and execute, it is 4+1=5.

The syntax of chmod command is 

chmod [options] mode filename

THe important options are:

-R : recursively change the permissions of a directory.
-v : Verbose

Chmod Examples in Linux / Unix: 

1. Give read, write and execute permissions to everyone. 

Read, write and execute: 4+2+1=7

$ chmod 777 sample.sh

In the above example, you can see that the permissions are specified with a three digit number. The first digit is for user permissions, second is for group and third is for others permission. This type of representation is called octal representation. Alternatively, you can use the symbolic representation to give the permissions.

chmod ugo+rwx sample.sh

We will see more details about the symbolic representation later.

2. Give read permission to user, write permission to group and execute permission to others. 

$ chmod 421 sample.sh

3. Recursive permissions to directory 

To give read and write permissions to all the users to a directory (including files and subdirectories) use the recursive option -R.

chmod -R 666 /dir

Symbolic Representation of Permissions: 

The following symbols are used to represent the users, groups and others:

u : User
g : Group
o : Others a : All (user, group and others)

The following symbols represent the permissions:

r : read
w : write
x : execute

The following symbols represent the permissions grant or revoke:

+ : Additional permissions. Selected permissions are added.
- : Revoke the permissions. Selected permissions are revoked.
= : Specific permissions. Only selected permissions are assigned.

Examples: 

1. Remove write permission from group 

$ chmod g-w sample.sh

This will only removes the write permission for the group.

2. Add new permission execute to others 

$ chmod o+x sample.sh

In addition to the existing permissions, this will add execute permission to others.

3. Give only read permissions to the user 

$ chmod u=w sample.sh

This will remove the existing permissions to the user and gives only write permission to the user.

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