Most device files will already be created and will be there ready to use after you install your Linux system. If by some chance you need to create one which is not provided then you should first try to use the MAKEDEV script. This script is usually located in /dev/MAKEDEV but might also have a copy (or a symbolic link) in /sbin/MAKEDEV. If it turns out not to be in your path then you will need to specify the path to it explicitly.
In general the command is used as:
# /dev/MAKEDEV -v ttyS0 create ttyS0 c 4 64 root:dialout 0660
ttyS0 is a serial port. The major and minor node numbers are numbers understood by the kernel. The kernel refers to hardware devices as numbers, this would be very difficult for us to remember, so we use filenames. Access permissions of 0660 means read and write permission for the owner (root in this case) and read and write permission for members of the group (dialout in this case) with no access for anyone else.
MAKEDEV is the preferred way of creating device files which are not present. However sometimes the MAKEDEV script will not know about the device file you wish to create. This is where the mknod command comes in. In order to use mknod you need to know the major and minor node numbers for the device you wish to create. The devices.txt file in the kernel source documentation is the canonical source of this information.
To take an example, let us suppose that our version of the MAKEDEV script does not know how to create the /dev/ttyS0 device file. We need to use mknod to create it. We know from looking at the devices.txt that it should be a character device with major number 4 and minor number 64. So we now know all we need to create the file.
# mknod /dev/ttyS0 c 4 64 # chown root.dialout /dev/ttyS0 # chmod 0644 /dev/ttyS0 # ls -l /dev/ttyS0 crw-rw---- 1 root dialout 4, 64 Oct 23 18:23 /dev/ttyS0
More information on what hardware resources the kernel is using can be found in the /proc directory. Refer to Section 3.7 in chapter 3.