This section describes how to configure
GRUB 2 for serial communications on machines with no display or keyboard.
To access the
GRUB 2 terminal over a serial connection, an additional option must be added to a kernel definition to make that particular kernel monitor a serial connection.
console=ttyS0 is the serial terminal to be used,
9600 is the baud rate,
n is for no parity, and
8 is the word length in bits. A much higher baud rate, for example
115200, is preferable for tasks such as following log files.
26.9.1. Configuring GRUB 2 for a single boot
To set the system to use a serial terminal only during a single boot process, when the GRUB 2 boot menu appears, move the cursor to the kernel you want to start, and press the e key to edit the kernel parameters. Remove the
quiet parameters and add console parameters at the end of the
linux16 line as follows:
linux16 /vmlinuz-3.10.0-0.rc4.59.el7.x86_64 root=/dev/mapper/rhel-root ro rd.md=0 rd.dm=0 rd.lvm.lv=rhel/swap crashkernel=auto rd.luks=0 vconsole.keymap=us rd.lvm.lv=rhel/root console=ttyS0,9600
These settings are not persistent and apply only for a single boot.
26.9.2. Configuring GRUB 2 for a persistent change
To make persistent changes to a menu entry on a system, use the
tool. For example, to update the entry for the default kernel, enter a command as follows:
grubby --remove-args="rhgb quiet" --args=console=ttyS0,9600 --update-kernel=DEFAULT
parameter also accepts the keyword
or a comma separated list of kernel index numbers. See the section called “Adding and Removing Arguments from a GRUB 2 Menu Entry”
for more information on using
26.9.3. Configuring a new GRUB 2 file
If required to build a new GRUB 2 configuration file, add the following two lines in the
GRUB_SERIAL_COMMAND="serial --speed=9600 --unit=0 --word=8 --parity=no --stop=1"
The first line disables the graphical terminal. Note that specifying the
GRUB_TERMINAL key overrides values of
GRUB_TERMINAL_OUTPUT. On the second line, adjust the baud rate, parity, and other values to fit your environment and hardware. A much higher baud rate, for example
115200, is preferable for tasks such as following log files. Once you have completed the changes in the
/etc/default/grub file, it is necessary to update the GRUB 2 configuration file.
grub.cfg file by running the
grub2-mkconfig command as follows:
On BIOS-based machines, issue the following command as
grub2-mkconfig -o /boot/grub2/grub.cfg
On UEFI-based machines, issue the following command as
grub2-mkconfig -o /boot/efi/EFI/redhat/grub.cfg
26.9.4. Using screen to Connect to the Serial Console
The screen tool serves as a capable serial terminal. To install it, run as
yum install screen
To connect to your machine using the serial console, use a command in the follow format:
screen /dev/console_port baud_rate
By default, if no option is specified, screen
uses the standard 9600 baud rate. To set a higher baud rate, enter:
, and so on.
To end the session in screen, press Ctrl+a, type
:quit and press Enter.
screen(1) manual page for additional options and detailed information.