{"id":7150,"date":"2019-01-22T23:11:18","date_gmt":"2019-01-22T22:11:18","guid":{"rendered":"https:\/\/pi3g.com\/?p=7150"},"modified":"2019-01-27T19:05:39","modified_gmt":"2019-01-27T18:05:39","slug":"debugging-the-alpine-boot-process","status":"publish","type":"post","link":"https:\/\/pi3g.com\/de\/debugging-the-alpine-boot-process\/","title":{"rendered":"Fehlersuche im Alpine-Boot-Prozess"},"content":{"rendered":"

As discussed in my previous post<\/a>, Alpine Linux goes through several stages when it boots.<\/p>\n

Just after mounting the boot media, and scanning it for apkovl\u2019s (with nlplug-findfs), there is an option for you to get a console, by setting $SINGLEMODE to yes.<\/p>\n

Setting SINGLEMODE to yes is easy, you simply add the word \u201csingle<\/strong>\u201d to the cmdline.txt<\/span> (if on Raspberry Pi) \/ your kernel parameters (if not on Raspberry Pi):<\/p>\n

modules=loop,squashfs,sd-mod,usb-storage single<\/strong> noquiet dwc_otg.lpm_enable=0 console=tty1<\/p><\/blockquote>\n

Note that I have also prepended the word quiet with no to effectively disable it \u2013 you will get more output during boot. (NB: this is actually the way this is meant to be done, as defined in the init script)<\/p>\n

Once you boot, this will drop you into a shell session (with ash, the built-in shell of busybox).<\/p>\n

Note: if you have set the root kernel parameter, to specify a root filesystem to boot from, your console is mounted before <\/strong>the root partition\u00a0 is mounted \u2013 probably so you can better debug it yourself.<\/p>\n

This single mode is very useful for trying boot configurations and debugging boot problems, and experimenting with the system and it\u2019s capabilities (which come entirely <\/strong>from the initramfs and the kernel).<\/p>\n

can\u2019t access tty error<\/h2>\n

The following error is thrown when Single mode is enabled:<\/p>\n

sh: can’t access tty: job control turned off<\/p><\/blockquote>\n

It seems to be related to sh\u2019s capabilities, and how it is executed \u2013 no need to be alarmed.<\/p>\n

USB keyboard doesn\u2019t work<\/h2>\n

At first, the USB keyboard didn\u2019t work for me; I am not sure what the reason for this is. After several power cycles it started working. Maybe it can\u2019t be switched in via KVM.<\/del><\/p>\n

In any case, keep trying, it should work. Try to type immediately when the sh warning comes \u2026<\/del><\/p>\n

On the Pi 3B+ the keyboard does not work; it only works on a Pi 3A+. I suspect this is due to the additional internal USB Hub on the Pi 3B+ – please try to use a Pi 3A+ to debug in single mode. (I know, not ideal for networking setups).<\/p>\n

Is there a shorter way to activate single mode? I am lazy<\/h2>\n

These are your options:<\/p>\n

s|single|1<\/p><\/blockquote>\n

So you should be able to get into single mode by adding an \u201cs\u201d or a \u201c1\u201d to the cmdline.txt. Not tested by me.<\/p>\n

 <\/p>\n

How can I exit singlemode?<\/h2>\n

Type<\/p>\n

exit<\/p><\/blockquote>\n

 <\/p>\n

What happens after the single mode check?<\/h2>\n

Our answer refers only <\/strong>to the RAM (\u201cdiskless\u201d) boot mode of Alpine Linux.<\/p>\n

The following operations are done only after the single mode check, so you will have to run them manually if you want:<\/p>\n