kernel boot parameter : init=/bin/bash

[dan]

James Wilkinson wrote:
> lin q wrote:
> 
>> I happen to find an interesting thing: if I add "init=/bin/bash" to the 
>>kernel boot parameter, then Fedora Core 1 quickly boots to command line 
>>bash, no GUI starts. And / is read only. And I works in single user 
>>environment.
>>
>> Then I tried changing the parameter to "init=/bin/bash, rw", but this time 
>>the boot is like normal way, GUI starts, I am asked to log in, etc.
>>
>> I wonder why it has so big difference with and without "rw"?
> 
> 
> Whew. There's quite a lot to explain here.
> 
> In Unix-like systems, init is the first process to be run, and the
> ultimate ancestor of all processes ever run. It's responsible for
> running all the init scripts.
> 
> You're telling the Linux kernel to run /bin/bash as init, rather than
> the system init. This means *nothing* else is running: bash doesn't know
> it has to start anything. bash won't do a number of other init-type
> things either, so don't expect everything to work.
> 
> Now, the kernel expects spaces to separate options on its command line,
> not commas. This means that in your second example, the kernel thinks it
> has two parameters, "init=/bin/bash," (*including* the comma, excluding
> the quotes) and "rw".
> 
> I don't have a 2.4 kernel tree any more, and I'm not a "C" programmer,
> but in 2.6.11, the kernel source file init/main.c includes these lines:
>     /*
>      * We try each of these until one succeeds.
>      *
>      * The Bourne shell can be used instead of init if we are
>      * trying to recover a really broken machine.
>      */
> 
>     if (execute_command)
>         run_init_process(execute_command);
> 
>     run_init_process("/sbin/init");
>     run_init_process("/etc/init");
>     run_init_process("/bin/init");
>     run_init_process("/bin/sh");
> 
>     panic("No init found.  Try passing init= option to kernel.");
> 
> So the kernel looks for "/bin/bash,", doesn't find it, and tries
> /sbin/init as normal. That works, and init (and Fedora) loads as usual.
> 
> Hope this helps,
> 
> James.
> 

Hey, that's some good information.  I never knew *exactly* where the 
kernel looks to start init scripts.

Thanks
-dant