[Qemu-devel] Re: Stop using "which" in ./configure

[Paolo Bonzini <pbonzini@redhat.com>]

On 01/20/2010 02:49 PM, Loïc Minier wrote:
> On Wed, Jan 20, 2010, Paolo Bonzini wrote:
>> On 01/20/2010 12:37 PM, Loïc Minier wrote:
>>> +    # not found
>>> +    IFS="$local_ifs"
>> If you do this, you should set IFS to space-tab-lf at the beginning of
>> the script, like this:
>>
>> IFS=" ""	""
>> "
>
>   Are you saying that I can't backup/restore IFS without setting it
>   first?

Yes, it affects the behavior of read for example:

$ echo a b c | (read a b c; echo $a; echo $b; echo $c)
a
b
c
$ IFS=
$ echo a b c | (read a b c; echo $a; echo $b; echo $c)
a b c


$

(It's not used by QEMU's configure, but it's better to be defensive).

>> or this (better because it does not rely on embedding whitespace
>> characters within the line):
>> IFS=`printf ' \t'`"
>> "
>
>   If we go that route, perhaps IFS="`printf ' \t\n'`" would be more
>   readable?  I'm not sure how common printf is though.

I tried that, but backtick strips trailing newlines (or something like 
that but anyway it does not work).  IFS=`printf ' \n\t'` would work (the 
double quotes are not needed) but the Autoconf manual suggests 
space-tab-newline in that order (even though only the leading space 
seems important from the rest of the paragraph).

printf is fine, though it may not be a builtin.  It's actually more 
portable (even if slower on some shells) to use printf than echo if you 
have variable substitutions in the string, because it handles 
consistently the case when the output starts with a minus sign.

Paolo