Working with the serial console

An Arch Linux machine can be configured for connections via the serial console port, which enables administration of a machine even if it has no keyboard, mouse, monitor, or network attached to it.
Installation of Arch Linux is possible via the serial console as well.
A basic environment for this scenario is two machines connected using a serial cable (9-pin connector cable).
The administering machine can be any Unix/Linux or Windows machine with a terminal emulator program (PuTTY or Minicom, for example).
The configuration instructions below will enable
boot loader
menu selection, boot messages, and terminal forwarding to the serial console.
Configure console access on the target machine
Boot loader
GRUB
When using
GRUB
with a generated
grub.cfg
, edit
/etc/default/grub
and enable serial input and output support:
/etc/default/grub
...
GRUB_TERMINAL_INPUT="console serial"
...
GRUB_TERMINAL_OUTPUT="gfxterm serial"
...
Next add the
GRUB_SERIAL_COMMAND
variable and set the options for the serial connection. For COM1 (
/dev/ttyS0
) with baud rate of 115200 bit/s:
/etc/default/grub
...
GRUB_SERIAL_COMMAND="serial --unit=0 --speed=115200"
Read GRUB's manual on
Using GRUB via a serial line
and the
serial command
for detailed explanation of the available options.
GRUB first stage serial console
When GRUB is installed to an encrypted /boot/ partition - GRUB first stage (core.img) will show
Enter passphrase for hdX,gptY:
password prompt only on platform console and will not show anything in serial console even if all serial console configuration steps were done properly.
This happens because the
grub-install
has special behavior for
GRUB_ENABLE_CRYPTODISK=y
and will create early config placed in
/boot/grub/PLATFORM/load.cfg
but
grub-install
ignores serial console configuration from
/etc/default/grub
for GRUB first stage.
The
/boot/grub/PLATFORM/load.cfg
file gets overwritten each time
grub-install
is launched.
To get the GRUB cryptodisk password prompt on the serial console a few extra configuration steps are required:
1. Run the following command to generate
/boot/grub/PLATFORM/load.cfg
and see the correct
grub-mkimage
parameters for your system:
EFI platforms:
# grub-install --verbose 2>&1 | grep grub-mkimage
BIOS platorms:
# grub-install --verbose /dev/YOUR_BOOT_DISK 2>&1 | grep grub-mkimage
You can reboot the system to check if it boots properly, but you should save the output of the commahd above - it will be hecessary at step 4.
2. Copy
/boot/grub/PLATFORM/load.cfg
to
/boot/grub/PLATFORM/early-grub.cfg
3. Add the following lines at the beginning of
/boot/grub/PLATFORM/early-grub.cfg
(change those lines according to desired configuration as described above):
serial --unit=0 --speed=115200
terminal_input serial console
terminal_output serial console
4. Edit
grub-mkimage
parameters from step 1 (the
grub-install
output):
Replace
--config '/boot/grub/PLATFORM/load.cfg'
to
--config '/boot/grub/PLATFORM/early-grub.cfg'
Remove paramters with empty arguments (like
--dtb ''
and
--sbat ''
) and add
serial
and
terminal
modules to the end of the
grub-mkimage
parameters list.
5. Run
grub-mkimage
with these parameters.
6. On BIOS platforms like
i386-pc
(for EFI platforms just skip this step) run the following command to install new
core.img
to your system:
# grub-bios-setup -d /boot/grub/i386-pc/ /dev/YOUR_BOOT_DISK
In order to revert those changes - just reinstall GRUB using
grub-install
command.
GRUB Legacy
Edit the
GRUB Legacy
configuration file
/boot/grub/menu.lst
and add these lines to the general area of the configuration:
serial --unit=0 --speed=9600
terminal --timeout=5 serial console
Note
When the
terminal --timeout=5 serial console
line is added to your
menu.lst
, your boot sequence will now show a series of
Press any key to continue
messages. If no key is pressed, the boot menu will appear on whichever (serial or console) appears first in the
terminal
configuration line.
rEFInd
rEFInd
supports serial console only in text mode. Edit
refind.conf
and uncomment
textonly
.
Syslinux
To enable serial console in
Syslinux
, edit
syslinux.cfg
and add
SERIAL
as the first directive in the configuration file.
For COM1 (
/dev/ttyS0
) with baud rate of 115200 bit/s:
SERIAL 0 115200
The serial parameters are hardcoded to 8 bits, no parity and 1 stop bit.
[1]
. Read
Syslinux Wiki:Config#SERIAL
for the directive's options.
Kernel
Kernel's output can be sent to serial console by setting the
console=
kernel parameter
. The last specified
console=
will be set as
/dev/console
.
console=tty0 console=ttyS0,115200
See
https://docs.kernel.org/admin-guide/serial-console.html
.
getty
At boot,
systemd-getty-generator(8)
will start a
getty
instance for each console specified in the kernel command line.
If you have not configured
console=
in kernel command line
start
serial-getty@
device
.service
. For
/dev/ttyS0
(COM1) that would be
serial-getty@ttyS0.service
.
Enable
the service to start it at boot.
Unless specified otherwise in the kernel command line, getty will be expecting 38400 bit/s baud rate, 8 data bits, no parity and one stop bit-times.
Making Connections
Connect using a terminal emulator program
Note
Before making a connection, it is recommended to add yourself to the
uucp
user group
. Otherwise you will need root permission to make a connection.
Perform these steps on the machine used to connect the remote console.
cu
uucp
tool
cu
can be used to "Call Up" another system and act as a serial console:
$ cu --line /dev/ttyS0
dterm
dterm-git
AUR
is a tiny serial communication program. If you invoke it without parameters, it will connect to
/dev/ttyS0
at 9600 baud by default. The following example connect to
/dev/ttyS0
at 115200 baud, with 8 data bits, no parity bit and 1 stop bit-times:
$ dterm 115200 8 n 1
See its
README
for more examples.
Minicom
minicom
can be obtained from the official repositories. Start Minicom in setup mode:
$ minicom -s
Using the textual navigation menu, change the serial port settings to the following:
Serial Device: /dev/ttyS0
Bps/Par/Bits: 9600 8N1
Press Enter to exit the menus (pressing Esc will not save changes).
Remove the modem Init and Reset strings, as we are not connecting to a modem. To do this, under the
Modem and Dialing
menu, delete the Init and Reset strings. Optionally save the configuration by choosing
save setup as dfl
from the main menu.
Restart minicom with the serial cable connected to the target machine.
The special keys for navigating Minicom can be found in the help menu which is opened by pressing
Ctrl+A Z
(i.e., exit session is
Ctrl+A X
).
picocom
picocom
is a tiny dumb-terminal emulation program that is very like minicom, but instead of
mini
, it is
pico
. The following example connect to
ttyS0
at 9600 bps:
$ picocom -b 9600 /dev/ttyS0
Note
if the backspace key will not work properly try out this option:
--omap delbs
See its manual for detailed usage.
Screen
GNU Screen
is able to connect to a serial port. It will connect at 9600 baud by default:
$ screen /dev/ttyS0
A different baud rate (e.g. 115200) may be specified on the command line.
$ screen /dev/ttyS0 115200
To end the session, press
Ctrl+a
followed by
K
. Alternatively, press
Ctrl+a
, type
:quit
and confirm it by pressing
Enter
.
Serialclient
Serialclient
[2]
is a CLI client for serial connection written in ruby. Install
ruby
package, then install it with the following:
# gem install serialclient
Then, you can use like this:
$ serialclient -p /dev/ttyS0
tinyserial
tinyserial
AUR
is a
minicom
replacement for accessing serial ports on Linux inspired by FreeBSD 'tip'.
$ com /dev/ttyS0 9600
tio
tio
AUR
is a simple serial device tool which features a straightforward command-line and configuration file interface to easily connect to serial TTY devices for basic I/O operations. It has less focus on classic terminal/modem features and more focus on the needs of embedded developers and hackers. tio was originally created to replace screen for connecting to serial devices when used in combination with tmux.
$ tio /dev/ttyUSB0
socat
socat
is a command line based utility that establishes two bidirectional byte streams and transfers data between them.
$ socat - /dev/ttyACM0,raw,echo=0,b115200
microcom
microcom Copy bytes from stdin to TTY and from TTY to stdout, its part of
busybox
$ busybox microcom -s 115200 /dev/ttyACM0
Graphical front-ends
cutecom
— A GUI enabled serial monitor.
https://gitlab.com/cutecom/cutecom
||
cutecom
AUR
PuTTY
— A terminal integrated SSH/Telnet client.
https://www.chiark.greenend.org.uk/~sgtatham/putty/
||
putty
moserial
— A GTK-based serial terminal, primarily intended for technical users and hardware hackers who need to communicate with embedded systems, test equipment, and serial consoles.
https://wiki.gnome.org/Apps/Moserial
||
moserial
gtkterm
— GTKTerm is a simple, graphical serial port terminal emulator for Linux and possibly other POSIX-compliant operating systems. It can be used to communicate with all kinds of devices with a serial interface, such as embedded computers, microcontrollers, modems, GPS receivers, CNC machines and more.
https://github.com/Jeija/gtkterm
||
gtkterm
AUR
Windows clients
On Windows machines, connect to the serial port using programs like
PuTTY
or
Terminalbpp
.
Installing Arch Linux using the serial console
Tip
The Arch Linux monthly release—the installation medium—boot loader
is configured
to listen
on
0
port—
ttyS0
or COM1,
at
115200
bps,
with 8 data bits,
with no parity bit,
with 1 stop bit-times.
Connect to the target machine using the method described above.
Boot the target machine using the Arch Linux installation medium.
When the boot loader appears, select
Boot Arch Linux (<arch>)
and press
Tab
to edit
Append
console=ttyS0,115200
and press
Enter
.
Now systemd should detect
ttyS0
and spawn a serial getty on it. Log in as
root
and start the installation as usual.
Note
After setup is complete, the console settings will not be saved on the target machine; in order to avoid having to connect a keyboard and monitor, configure console access on the target machine before rebooting.
While a port speed of
9600
is used in most of the examples in this document, working with higher values is recommended. List of available speeds is displayed in Minicom by pressing
Ctrl+a
and then
p
.
Debugging an unresponsive machine using a serial console
Even though
[3]
has only raw and terse instructions, it presents the full scene. It is important to note that here, the machine under test got unresponsive in a  reproducible manner. And that it happened during normal operation. So it could be accessed normally before it needed debugging. However, in general, the serial console is also useful for debugging boot issues. Perhaps by configuring the
boot loader
by hand at machine startup time. Also note the mentioned
netconsole
within the P.S paragraph of the external link from this section.
Tips and tricks
On-demand serial-getty for ttyUSB
Debugging a system by attaching a USB serial adapter requires
starting
serial-getty@ttyUSB
N
.service
which may not be possible if the system cannot be interacted with in normal ways. A solution is to write an
udev rule
that starts the service when a USB serial adapter is plugged in.
To allow using the USB serial adapter
the other way around
too, the udev rule can be limited to trigger only on a specific USB port. To do that, plug in the adapter and find out its used USB port identifier. E.g. for
/dev/ttyUSB0
run:
$ udevadm info --query=property --property=ID_PATH --value --name=/dev/ttyUSB0
Create
a udev rule with the following contents using the value obtained with the previous command:
/etc/udev/rules.d/99-usb_serial-getty.rules
ACTION=="add", SUBSYSTEM=="tty", ENV{ID_BUS}=="usb", ENV{ID_PATH}=="
your_ID_PATH_value
", TAG+="systemd", ENV{SYSTEMD_WANTS}+="serial-getty@$kernel.service"
Re-plug the USB serial adapter and the appropriate
serial-getty@ttyUSB
N
.service
will get started automatically. Verify it by checking the unit status with
systemctl
. The service will get automatically stopped when the USB device is unplugged.
Troubleshooting
Ctrl+c and Minicom
If you are having trouble sending a
Ctrl+c
command through minicom you need to switch off hardware flow control in the device settings (
minicom -s
), which then enables the break.
RS-232 barcode scanners (garbled output)
Some barcode scanners connect via RS-232 using USB-to-serial adapters (e.g. FTDI) and appear as
/dev/ttyUSB*
devices. These scanners output barcode data as a byte stream and do not use the SANE scanning stack.
If scanned barcodes produce unreadable characters (such as the Unicode replacement character
�
), this is commonly caused by a mismatch in serial framing parameters rather than an incorrect baud rate.
Many scanners default to 7 data bits with even parity (7E1), while Linux tools often default to 8 data bits with no parity (8N1). To test and correct this, explicitly configure the serial port:
stty -F /dev/ttyUSB0 9600 cs7 parenb -parodd -cstopb -ixon -ixoff clocal
cat /dev/ttyUSB0
Resizing a terminal
Unlike ssh, serial connections do not have a mechanism to transfer something like
SIGWINCH
when a terminal is resized. This can cause weird problems with some full-screen programs (e.g.
less
) when you resize your terminal emulator's window.
Resizing the terminal via
stty
is a workaround:
$ stty rows
lines
cols
columns
However, this requires you to manually input the proper geometry. The following methods should be simpler.
1. There is a lesser-known utility called
resize
, shipped with
xterm
, that can solve this problem. Invoke it without parameters after you resize the terminal emulator's window:
$ resize
2. If you do not want to install xterm, it is possible to do the same work via a shell function. Put the following function into your bash/zshrc and invoke it without parameters after resizing the terminal emulator's window:
rsz() {
	if [[ -t 0 && $# -eq 0 ]];then
		local IFS='[;' escape geometry x y
		echo -ne '\e7\e[r\e[999;999H\e[6n\e8'
		read -t 5 -sd R escape geometry || {
			echo unsupported terminal emulator. >&2
			return 1
		}
		x="${geometry##*;}" y="${geometry%%;*}"
		if [[ ${COLUMNS} -eq "${x}" && ${LINES} -eq "${y}" ]];then
			echo "${TERM} ${x}x${y}"
		elif [[ "$x" -gt 0 && "$y" -gt 0 ]];then
			echo "${COLUMNS}x${LINES} -> ${x}x${y}"
			stty cols ${x} rows ${y}
		else
			echo unsupported terminal emulator. >&2
			return 1
		fi
	else
		echo 'Usage: rsz'
	fi
}
Extra hardware serial ports
The generic 8250 serial driver exposes 32 hardware serial ports, as set in the Arch kernel configuration. This means by default serial ports are created numbered from
/dev/ttyS0
to
/dev/ttyS31
. On most systems many of these ports will be non-functional.
The number can be reduced by setting the
kernel parameter
8250.nr_uarts
. E.g.:
8250.nr_uarts=5
This value must be set as a kernel boot parameter not a module option, as the
serial8250
driver is compiled into the kernel image.