從Systemd(Systemctl)設置：
$ cp /usr/lib/systemd/system/serial-getty@.service /etc/systemd/system/serial-getty@ttyS0.service
$ vi /etc/systemd/system/serial-getty@ttyS0.service
//# Modify "ExecStart" line as below.
//# E.g.: Rate=57600,
>> ExecStart=-/sbin/agetty 57600 %I $TERM
//# E.g.: Rate=115200,
>> ExecStart=-/sbin/agetty 115200 %I $TERM
//# E.g.: Rate=115200,
>> ExecStart=-/sbin/agetty 115200 %I $TERM
//# E.g.: Rate=115200,
>> ExecStart=-/sbin/agetty -o '-p -- \\u' 115200 %I $TERM
//# Recommend remove "--keep-baud" that if you expect it take effect immediately.(Mean without reboot or others operation.)
$ systemctl daemon-reload
//# For a built in serial port /dev/ttyS0
$ systemctl enable --now serial-getty@ttyS0.service
$ systemctl status serial-getty@ttyS0.service
從GRUB設置( E.g.:RHEL/CentOS EL7以後 或 Ubuntu)
//# Modify "/etc/default/grub" file and add "console=tty0 console=ttyS0,115200n8" in "GRUB_CMDLINE_LINUX" line.
$ vi /etc/default/grub
//# Before modify: GRUB_CMDLINE_LINUX=""
>> GRUB_CMDLINE_LINUX="console=tty0 console=ttyS0,115200n8"
//# Keep original parameters, just add 'console=tty0 console=ttyS0,115200n8'.
//# (Not necessary) From some information on Internet, it will also add "serial --unit=1 --speed=115200 --word=8 --parity=no --stop=1" in "GRUB_SERIAL_COMMAND" line.
$ vi /etc/default/grub
//# Add: 'serial --unit=1 --speed=115200 --word=8 --parity=no --stop=1'
>> GRUB_SERIAL_COMMAND="serial --unit=1 --speed=115200 --word=8 --parity=no --stop=1"
//# If under OS contain file: /etc/securetty
>> insert ttyS0
//# Update GRUB
//# If OS: Ubuntu. Update grub.cfg:
$ grub-mkconfig -o /boot/grub/grub.cfg"
//# If OS: RHEL/CentOS/ Enterprise Linux series(EL x) OS.
//$ Update grub.cfg for Legacy OS:
$ grub2-mkconfig -o /boot/grub2/grub.cfg
//# Update grub.cfg for UEFI BIOS:
$ grub2-mkconfig -o /boot/efi/EFI/xxxxx/grub.cfg"
//# If RHEL OS, xxxxx=redhat. If CentOS, xxxxxx=centos , ...etc "
兩者的差異是：
1. 從Systemd(Systemctl) 啟用：OS開機至user login才會將文字訊息會顯示在Serial上
意思是 BIOS開機脫離BIOS POST到Linux login之間的任何階段，Serial的畫面都不會有文字顯示出來
E.g. Server BIOS:
Configuring firmware is in progress...
Importing firmware configuration settings from OData server.
Wait for BMC to update Inventory data > \
Ubuntu 22.04 LTS localhost ttyS0
2. 從GRUB 啟用：開機過程／關機過程的文字訊息會顯示在Serial上
E.g. Server BIOS:
Configuring firmware is in progress...
Importing firmware configuration settings from OData server.
Wait for BMC to update Inventory data > \
(xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx OS 啟用過程的訊息 xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx)
(xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx OS 啟用過程的訊息 xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx)
(xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx OS 啟用過程的訊息 xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx)
(xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx OS 啟用過程的訊息 xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx)
Ubuntu 22.04 LTS localhost ttyS0
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