handling script/init errors

[Sascha Hauer]

On Tue, Nov 12, 2013 at 08:05:19PM +1100, Marc Reilly wrote:
> Hi Sasha,
> 
> > > 
> > > Is there a way to set up an error handler in the scripts? Ideally, a
> > > command or script that could be called if /bin/sh encounters an error.
> > 
> > What would you consider an error? Is executing the 'false' command an
> > error?
> > Commands in scripts must be allowed to fail. You are supposed to catch
> > this via
> > 
> > if [ $? != 0 ]; then
> > 	echo "something bad happened"
> > 	exit 1
> > fi
> 
> "Error" is really a terrible, non-specific word. Sorry. 
> And to make it worse I'm not really sure what the error, um, problem, truly 
> is...
> 
> I scattered a few "false" commands in the init script and it continued onto 
> the end, but when i added (on a board where there there is no bus #1):
> 
> {{{
> # force reset audio dac in case audio playing during soft reset
>  i2c_write -b 1 -a 0x47 -r 0x55 0x80
> }}}
> 
> This causes the init script to just stop and drop to a prompt, assuming 
> because bus #1 was not available. I haven't looked into how the errors/ return 
> codes are different.

hush used to interpret return values from commands < 0 as 'exit'. This
changed with this commit:

| commit 16edced39ecf4c316179b72c01af249f85b36218
| Author: Sascha Hauer <s.hauer at pengutronix.de>
| Date:   Fri Aug 10 12:40:01 2012 +0200
| 
|     hush: Make exit a shell builtin
|     
|     'exit' used to do its job by returning value < 0. This is a sign
|     for hush that 'exit' is executed. This has problems:
|     
|     - Often commands accidently return a negative value. This causes
|       the shell to exit.
|     - execute_binfmt returns a negative value when it does not find
|       a binary to execute. This again causes the shell to exit.
|       Returning a negative error value seems to be the right thing
|       to do, but catching this in the shell would mean that the exit
|       command does not work anymore.
|     - if called without arguments exit is supposed to return the code
|       of the last command. As a command exit has no access to this code.
|     
|     This patch changes exit to be a builtin and also fixes the last return
|     code problem. While at it, update the help text.
|     
|     Signed-off-by: Sascha Hauer <s.hauer at pengutronix.de>

Probably the patch above introduced problems of its own fixed in later
commits, so if your device is in production you're better off looking
at i2c_write. I assume it returns a negative error value under some
circumstances. Let it return 1 instead.

Sascha

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