Chmod

Chmod is used to manage permissions (mode) on a file or directory

The syntax of the chmod command when using the symbolic mode has the following format:

chmod [OPTIONS] NUMBER FILE...

When using the numeric mode, you can set the permissions for all three user classes (owner, group, and all others) at the same time.

The permission number can be a 3 or 4-digits number. When 3 digits number is used, the first digit represents the permissions of the file’s owner, the second one the file’s group, and the last one all other users.

Each write, read, and execute permissions have the following number value:

• r (read) = 4
• w (write) = 2
• x (execute) = 1
• no permissions = 0

The permissions number of a specific user class is represented by the sum of the values of the permissions for that group.

To find out the file’s permissions in numeric mode, simply calculate the totals for all users’ classes. For example, to give read, write and execute permission to the file’s owner, read and execute permissions to the file’s group and only read permissions to all other users, you would do the following:

• Owner: rwx=4+2+1=7
• Group: r-x=4+0+1=5
• Others: r-x=4+0+0=4

Using the method above, we come up to the number 754, which represents the desired permissions.

When the 4 digits number is used, the first digit has the following meaning:

• setuid=4
• setgid=2
• sticky=1
• no changes = 0

The next three digits have the same meaning as when using 3 digits number.

If the first digit is 0 it can be omitted, and the mode can be represented with 3 digits. The numeric mode 0755 is the same as 755.

To calculate the numeric mode, you can also use another method (binary method), but it is a little more complicated. Knowing how to calculate the numeric mode using 4, 2, and 1 is sufficient for most users.

You can check the file’s permissions in the numeric notation using the stat command:

stat -c "%a" file_name

Here are some examples of how to use the chmod command in numeric mode:

• Give the file’s owner read and write permissions and only read permissions to group members and all other users:

chmod 644 dirname

• Give the file’s owner read, write and execute permissions, read and execute permissions to group members and no permissions to all other users:

chmod 750 dirname

• Give read, write, and execute permissions, and a sticky bit to a given directory:

chmod 1777 dirname

• Recursively set read, write, and execute permissions to the file owner and no permissions for all other users on a given directory:

chmod -R 700 dirname