How can i config rasp lite to open a shell on the serial uart on boot? Params are 1200-8-N-1 Dont want login running, just straight to sh
In this article, we’ll walk through one way of implementing this configuration.
Activate the serial port
Raspbian automatically starts a getty on the serial port if one is available. You should see an
agetty process associated with your serial port when you run
ps -ef. For example:
root@raspberrypi:/etc/systemd/system# ps -fe | grep agetty | grep ttyS0 root 1138 1 0 00:24 ttyS0 00:00:00 /sbin/agetty -o -p -- \u --keep-baud 115200,38400,9600 ttyS0 vt220
If you don’t see this process and you’re on a Raspberry Pi 3 (or later), you may need to explicitly enable the serial port by adding
/boot/config.txt. If you make this change, reboot your Pi before continuing, then repeat the above test to make sure things are working as expected.
Note that your serial port may not always be named
ttyS0. I’m going to use the value
ttyS0 throughout this article to represent the appropriate device name. The correct device name is the penultimate argument in the above
Modify the serial-getty@ttyS0 unit
agetty process we saw in the previous section is started by the
serial-getty@ttyS0.service service unit (which is an instance of the
serial-getty@.service template unit). We need to modify that service so that it will call
agetty with the
--autologin root option.
Rather than editing the unit file included in Raspbian, we’re going to make the changes by creating a systemd “drop-in” configuration to override the stock service unit. From the systemd.unit man page:
Along with a unit file foo.service, a “drop-in” directory foo.service.d/ may exist. All files with the suffix “.conf” from this directory will be parsed after the unit file itself is parsed. This is useful to alter or add configuration settings for a unit, without having to modify unit files.
The easiest way to creating a drop-in unit is with the
systemctl edit command:
systemctl edit serial-getty@ttyS0
This will bring up an editor (
nano by default, unless you have set
VISUAL in your environment to point at a different editor) in which you will create
Enter the following content:
[Service] ExecStart= ExecStart=/sbin/agetty -o '-p -- \u' --keep-baud 115200,38400,9600 --noclear --autologin root ttyS0 vt220
(That while the original request was for a getty running as 1200 bos, the above configuration is more generally useful. To allow connectoins at 1200 bps, modify the list of rates above to looking something like
115200,38400,9600,1200 (if you want to permit connections at higher speeds) or just
1200 (if you really want to permit only 1200 bps connections).
Save the file, then reload
systemd by running
systemctl daemon-reload. This tells
systemd to re-read its unit files.
Finally, restart the
systemctl restart serial-getty@ttyS0
Configure passwordless root login on the console
With the above change to the service unit,
agetty will attempt to log in the
root user on the console but will prompt for a password. That looks like:
Raspbian GNU/Linux 10 raspberrypi ttyS0 raspberrypi login: root (automatic login) Password:
We need to configure things such that the
root user does not require a password when logging on the serial console. We’ll do this by modifying the PAM configuration for the
Add the following to the top of
auth sufficient pam_listfile.so item=tty sense=allow file=/etc/rootshelltty onerr=fail apply=root
login to permit a login for the
root user if it finds the login tty in the file
Now, add the serial port device to
root@raspberrypi:/etc# echo /dev/ttyS0 > /etc/rootshelltty
These changes will take affect as soon as
agetty restarts. You can wait for the
Password: prompt to timeout, or just restart the service by running
systemctl restart serial-getty@ttyS0.
With these changes, the Pi will now automatically start a
root shell on the serial port without prompting for a password:
Raspbian GNU/Linux 10 raspberrypi ttyS0 raspberrypi login: root (automatic login) Last login: Mon Feb 24 00:29:00 EST 2020 on ttyS0 Linux raspberrypi 4.19.97-v7+ #1294 SMP Thu Jan 30 13:15:58 GMT 2020 armv7l [...] root@raspberrypi:~#