/dev/console exists primarily to expose the kernel’s console to userspace. The Linux kernel’s documentation on devices now says
The console device, /dev/console, is the device to which system messages should be sent, and on which logins should be permitted in single-user mode. Starting with Linux 2.1.71, /dev/console is managed by the kernel; for previous versions it should be a symbolic link to either /dev/tty0, a specific virtual console such as /dev/tty1, or to a serial port primary (tty*, not cu*) device, depending on the configuration of the system.
/dev/console, the device node with major 5 and minor 1, provides access to whatever the kernel considers to be its primary means of interacting with the system administrator; this can be a physical console connected to the system (with the virtual console abstraction on top, so it can use tty0 or any ttyN where N is between 1 and 63), or a serial console, or a hypervisor console, or even a Braille device. Note that the kernel itself doesn’t use /dev/console: devices nodes are for userspace, not for the kernel; it does, however, check that /dev/console exists and is usable, and sets init up with its standard input, output and error pointing to /dev/console.
As described here, /dev/console is a character device with a fixed major and minor because it’s a separate device (as in, a means of accessing the kernel; not a physical device), not equivalent to /dev/tty0 or any other device. This is somewhat similar to the situation with /dev/tty which is its own device (5:0) because it provides slightly different features than the other virtual console or terminal devices.
The “list of consoles” is indeed the list of consoles defined by the console= boot parameters (or the default console, if there are none). You can see the consoles defined in this way by looking at /proc/consoles. /dev/console does indeed provide access to the last of these:
You can specify multiple console= options on the kernel command line. Output will appear on all of them. The last device will be used when you open /dev/console.
Best Answer
Almost all the files under
/devare device files. Whereas reading and writing to a regular file stores data on a disk or other filesystem, accessing a device file communicates with a driver in the kernel, which generally in turn communicates with a piece of hardware (a hardware device, hence the name).There are two types of device files: block devices (indicated by
bas the first character in the output ofls -l), and character devices (indicated byc). The distinction between block and character devices is not completely universal. Block devices are things like disks, which behave like large, fixed-size files: if you write a byte at a certain offset, and later read from the device at that offset, you get that byte back. Character devices are just about anything else, where writing a byte has some immediate effect (e.g. it's emitted on a serial line) and reading a byte also has some immediate effect (e.g. it's read from the serial port).The meaning of a device file is determined by its number, not by its name (the name matters to applications, but not to the kernel). The number is actually two numbers: the major number indicates which driver is responsible for this device, and the minor number allows a driver to drive several devices¹. These numbers appear in the
ls -llisting, where you would normally find the file size. E.g.brw-rw---- 1 root disk 8, 0 Jul 12 15:54 /dev/sda→ this device is major 8, minor 0.Some device files under
/devdon't correspond to hardware devices. One that exists on every unix system is/dev/null; writing to it has no effect, and reading from it never returns any data. It's often convenient in shell scripts, when you want to ignore the output from a command (>/dev/null) or run a command with no input (</dev/null). Other common examples are/dev/zero(which returns null bytes ad infinitum)/dev/urandom(which returns random bytes ad infinitum).A few device files have a meaning that depends on the process that accesses it. For example,
/dev/stdindesignates the standard input of the current process; opening from has approximately the same effect as opening the original file that was opened as the process's standard input. Somewhat similarly,/dev/ttydesignates the terminal to which the process is connected. Under Linux, nowadays,/dev/stdinand friends are not implemented as character devices, but instead as symbolic links to a more general mechanism that allows every file descriptor to be referenced (as opposed to only 0, 1 and 2 under the traditional method); for example/dev/stdinis a symbolic link to/proc/self/fd/0. See How does /dev/fd relate to /proc/self/fd/?.You'll find a number of symbolic links under
/dev. This can occur for historical reasons: a device file was moved from one name to another, but some applications still use the old name. For example,/dev/scd0is a symbolic link to/dev/sr0under Linux; both designate the first CD device. Another reason for symbolic links is organization: under Linux, you'll find your hard disks and partitions in several places:/dev/sdaand/dev/sda1and friends (each disk designated by an arbitrary letter, and partitions according to the partition layout),/dev/disk/by-id/*(disks designated by a unique serial number),/dev/disk/by-label/*(partitions with a filesystem, designated by a human-chosen label); and more. Symbolic links are also used when a generic device name could be one of several; for example/dev/dvdmight be a symbolic link to/dev/sr0, or it might be a link to/dev/sr1if you have two CD readers and the second one is to be the default DVD reader.Finally, there are a few other files that you might find under
/dev, for traditional reasons. You won't find the same on every system. On most unices,/dev/logis a socket that programs use to emit log messages./dev/MAKEDEVis a script that creates entries in/dev. On modern Linux systems, entries in/dev/are created automatically by udev, obsoletingMAKEDEV.¹ This is actually no longer true under Linux, but this detail only matters to device driver writers.