device types

The first character in an ls -l mode identifies an object's filesystem type. Under /dev, character and block special files are device nodes. Pipes and Unix-domain socket nodes can also appear there, but they are interprocess communication objects rather than hardware device nodes.

$ ls -l /dev/null /dev/sda /run/systemd/journal/dev-log /tmp/example-fifo
crw-rw-rw- 1 root root 1, 3 ... /dev/null
brw-rw---- 1 root disk 8, 0 ... /dev/sda
srw-rw-rw- 1 root root      ... /run/systemd/journal/dev-log
prw------- 1 user user      ... /tmp/example-fifo

Entries and permissions vary by system; the example illustrates type characters only.

Character Device Nodes

A c identifies a character device. It usually exposes a stream-oriented or device-specific interface rather than addressable fixed-size storage blocks. Examples include terminals and pseudo-devices such as /dev/null.

“Character” does not require each system call to transfer exactly one character. Applications can read or write buffers, while the driver defines blocking, framing, and control behavior.

Block Device Nodes

A b identifies a block device. Block devices provide addressable storage in blocks through the kernel's block layer and can support operations such as buffered I/O, partitioning, and filesystems. Disks, partitions, and logical volumes commonly have block nodes.

A block node is not a mounted filesystem. It represents a storage device or logical region; a filesystem can be created on it and mounted separately. Writing raw data to the wrong block node can destroy partition tables, filesystems, or user data.

FIFOs and Socket Nodes

A p identifies a FIFO, also called a named pipe. It provides a named byte stream through which processes can communicate. The data is not persistently stored in the FIFO node after being consumed.

An s identifies a Unix-domain socket node. It names a local socket endpoint and can support connection-oriented or datagram communication, descriptor passing, and peer credential features. Network sockets using Internet addresses do not necessarily have filesystem nodes.

Neither a FIFO nor a Unix socket node uses device major and minor numbers to select a hardware driver.

Major and Minor Device Numbers

Character and block device nodes store a device number split into major and minor components. In a long listing they replace the ordinary file-size column:

brw-rw---- 1 root disk 8, 0 ... /dev/sda

The pair tells the kernel which registered device interface and instance the node addresses. A major number is associated with a driver or device class, while the driver interprets the minor number. Do not hard-code assumptions such as “minor zero always means the first drive”; mappings depend on the subsystem and kernel interfaces.

Display type and device numbers explicitly with:

$ stat -c 'type=%F major=%t minor=%T path=%n' /dev/null

The %t and %T values are shown in hexadecimal by GNU stat.

Summary

You can now interpret special filesystem types without treating all of them as hardware devices.

1. Read c as character and b as block device nodes.
2. Read p as FIFO and s as Unix-domain socket node.
3. Associate major and minor numbers only with device nodes.
4. Treat raw block-device access as potentially destructive.