1. Introduction
In this tutorial, we’ll first explore the two tty drivers present in a Linux system; /dev/tty and /dev/console. Then we’ll discover the different roles of /dev/console, /dev/tty0 and /dev/tty.
2. /dev/console
/dev/console is known as the system console. Historically this was the terminal attached directly to the Linux computer. Now it provides an option to connect a serial terminal to a Linux computer.
Let’s have a look at /dev/console:
$ ls -lah /dev/console
crw------- 1 root root 5, 1 Jun 15 15:25 /dev/console
$ file /dev/console
/dev/console: character special (5/1)
As we can see, it’s a file that lives in the folder /dev. This represents a character device with an identifier consisting of a major number of 5 and a minor number of 1.
Looking at the kernel documentation, it states that /dev/console used to be a symbolic link. As a result, /dev/console is pointing to another device.
Let’s see what /dev/console points to:
$ cat /sys/devices/virtual/tty/console/active
tty0
Hmm, does this mean that tty0 and /dev/console are the same thing? Let’s investigate further.
If we look at the documentation for the Linux Serial Console it states that we can change what /dev/console points to.
Let’s try and do that:
Note: This example requires a GRUB bootloader.
$ cat /etc/default/grub | head -n 10
# If you change this file, run 'update-grub' afterwards to update
# /boot/grub/grub.cfg.
# For full documentation of the options in this file, see:
# info -f grub -n 'Simple configuration'
GRUB_DEFAULT=0
GRUB_TIMEOUT=0
GRUB_DISTRIBUTOR=`lsb_release -i -s 2> /dev/null || echo Debian`
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"
GRUB_CMDLINE_LINUX="ignore_loglevel"
We have to modify the line in /etc/default/grub with GRUB_CMD_LINE_LINUX=”ignore_loglevel” to:
GRUB_CMD_LINE_LINUX=”console=/dev/tty2 ignore_loglevel”
Let’s update the grub config and restart:
$ sudo update-grub
$ sudo reboot
Now let’s have a look at where /dev/console is pointing:
$ cat /sys/devices/virtual/tty/console/active
tty2
We can see that /dev/console has changed.
So now that we know we can change what /dev/console points to. What’s the purpose of this device?
It seems that /dev/console provides the kernel a guaranteed place to output its log messages, especially during boot time. Also, the device used when switching to single-user mode is /dev/console.
Let’s give this a go by entering into single-user mode:
$ init 1
We notice that the graphics have disappeared. We’re in single-user text mode!
Let’s see which tty we are on:
$ tty
tty2
Which is the same as /dev/console.
As we’ve discovered. /dev/console can be pointed to a variety of devices. Kernel messages are logged to /dev/console where a login prompt can be launched.
3. /dev/tty0
/dev/tty0 points to the active tty terminal. Let’s explore further with some examples.
To access a tty3 lets press CTRL + ALT + F3. We can confirm that we’re using tty3 by typing in the command tty:
$ tty
/dev/tty3
Let’s see if we can write to the console using echo:
$ echo "hello world" > /dev/tty3
hello world
Okay, so far we’ve seen that we can change to a tty N by pressing CTRL + ALT + FN. Note however that there is not F0 key on the keyboard but there is a tty0.
Let’s try and find a bit more about tty0:
$ cat /sys/devices/virtual/tty/tty0/active
tty3
Seems like tty0 is pointing to tty3. Okay, let’s try and switch to a separate tty. Let’s say tty4. A quick shortcut would be to press CTRL + ALT + Right Arrow. This would take us from tty3 to tty4.
$ tty
/dev/tty4
$ cat /sys/devices/virtual/tty/tty0/active
tty4
We can see that tty0 is the current active console.
4. /dev/tty
In this section, we’ll explore how /dev/tty is different from /dev/tty0. /dev/tty is an alias to the terminal of the active process, regardless of the type of terminal. For instance, this can be the terminal associated with bash or sshd.
According to the POSIX standard, /dev/tty is considered to be the terminal associated with a process group whereas /dev/tty0 is associated with a virtual terminal.
Let’s see what that means in practice.
First, let’s ssh into the local machine and check our tty:
$ ssh localhost
Welcome to Ubuntu 20.04 LTS (GNU/Linux 5.4.0-37-generic x86_64)
* Documentation: https://help.ubuntu.com
* Management: https://landscape.canonical.com
* Support: https://ubuntu.com/advantage
0 updates can be installed immediately.
0 of these updates are security updates.
Last login: Tue Jun 16 10:59:08 2020 from 127.0.0.1
$ tty
/dev/pts/0
This is a pts, which is a different type of terminal. Now let’s check whether printing “hello world” to /dev/tty0 affects our ssh session:
Note that only root can write to /dev/tty0, therefore*,* we should switch to root:
$ sudo -i
$ echo "hello world" > /dev/tty0
$ # Nothing here was expecting hello world.
$ exit # let's leave root.
The message, “hello world” is not printed. If we check the active tty however, we should be able to find that “hello world” is printed there.
Let’s try using /dev/tty:
$ echo "hello world" > /dev/tty
hello world
The message, “hello world” is printed! This is because /dev/tty represents the terminal which is associated with the ssh session. Regardless of what terminal is in use, /dev/tty is a quick way to access the current terminal.
5. Conclusion
In this tutorial, we’ve looked at /dev/console and its role. We then looked at tty and tty0 and seen the difference between the two. /dev/tty is the current active tty and /dev/tty0 is a terminal associated with the current process.