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Special Files

The term "special file" on Linux/Unix means a file that can generate or receive data.
Since "everything is a file™," this includes storage devices, pipes, file descriptors, and character special files.

Common Special Files

  • /dev/null: A character special file that can receive data.
  • /dev/random: A character special file that can generate data.
  • /dev/sda: A block special file that can receive and generate data.

Creating Special Files with mknod

The mknod command is used to create block special files, character special files, or pipes (FIFOs/named pipes).

The term "special file" on Linux/Unix means a file that can generate or receive data.

Syntax: 

mknod [OPTION]... NAME TYPE [MAJOR MINOR]

  • NAME: The path to the special file.
  • TYPE: You can specify the TYPE of file:
    • p: Pipe (FIFO) special file.
      • Not sure why you'd want to use this over mkfifo, but here we are.
    • b: Block device special file. Requires major/minor numbers.
    • c: Character special file. Requires major/minor numbers.
  • MAJOR: Major device number (identifies the driver).
  • MINOR: Minor device number (identifies the specific device handled by that driver).

Also see Special File major and minor numbers.

Example: Creating a Null Device with mknod

ls -l /dev/null
# crw-rw-rw- 1 root root 1, 3 Mar  4 13:38 /dev/null
You can see the major and minor numbers of /dev/null are 1 and 3 respectively.
We can use those to duplicate the file with mknod.
sudo mknod /tmp/mynull c 1 3
sudo chmod 666 /tmp/mynull

  • This creates a character device at /tmp/mynull with major 1 and minor 3.
  • That matches the null device (/dev/null).

You can also save yourself the chmod and use the -m option: 

sudo mknod -m 666 /tmp/mynull c 1 3

mknod Options

There are two main options/flags for mknod.
One for permissions, the other for SELinux contexts.

  • -m/--mode=: Specify the mode (permissions) for the special file.
  • -Z/--context[=context]: Specify the SELinux context for the special file.
    • If -Z is used, it adjusts the SELinux context to the system default type for the destination file.
    • If the long option is used, but no context is given, it does the same as -Z.
    • If the long option is used and a context is given, it will set the context for the file.

Special File Major and Minor Numbers

When specifying either a block or character special file with mknod, you need to specify major and minor device numbers.

  • The MAJOR number tells the kernel which driver to use (e.g., the block driver).
    • This is like the "class" of device.
  • The MINOR number tells the kernel which instance of the device the file refers to.
    • This is the "specific device in that class."

You can check major and minor numbers with stat or ls -l.

stat /dev/sda
Output: 
  File: /dev/sda
  Size: 0               Blocks: 0          IO Block: 4096   block special file
Device: 5h/5d   Inode: 323         Links: 1     Device type: 8,0
Access: (0660/brw-rw----)  Uid: (    0/    root)   Gid: (    6/    disk)
Access: 2025-04-28 15:44:20.295315382 -0400
Modify: 2025-03-04 13:38:48.603526535 -0500
Change: 2025-03-04 13:38:48.603526535 -0500
 Birth: -
You can see the Device type: 8,0.
This is the major and minor number.

Or, using ls -l on a special file, you can see its major and minor numbers.

ls -l /dev/sda
# brw-rw---- 1 root disk 8, 0 Mar  4 13:38 /dev/sda
This is a block device (b) with a major number of 8 (driver: sd), and a minor number of 0 (/dev/sda).
/dev/sda1 would be minor number 1.