porter's handbook documentation on avoiding != variable assignments

[Matthew Seaman]

Kris Kennaway wrote:
> Can someone add to the PH a version of the advice below?  It might need 
> to be fleshed out a bit to provide more context.
> 
> Kris
> 
>> Variable assignments with != are bad!  Try as hard as you can to avoid 
>> using them -- especially in Mk/*!  Every time something processes your 
>> makefile it will spawn a command, even if it is not relevant for the 
>> operation being performed.  If you need to run shell commands, try to 
>> isolate them within a makefile target.  You can avoid code duplication 
>> by assigning the *shell commands* (not their output) to a variable and 
>> inserting it into your code block.
>>
>> e.g. instead of
>>
>> -- 
>> VARIABLE!=    do some shell stuff; do some other stuff
>>
>> target:
>>     echo ${VARIABLE}
>> -- 
>>
>> do this (or similar):
>>
>> -- 
>> VARIABLE_CMDS=    do some shell stuff; do some other stuff
>>
>> target:
>>     echo $$(${VARIABLE_CMDS})
>> -- 
>>
>> This defers the command execution to the point where the target runs, 
>> so in the case when the target is *not* run (e.g. during INDEX builds or
>> other recursive port traversals), then you avoid wasting one or more 
>> process executions.

Would it be acceptable to use something like:

.if !defined(BUILDING_INDEX)
VARIABLE!= something or other
.endif

or 

.if !make(describe)
VARIABLE!= whatever
.endif

	Cheers,

	Matthew

-- 
Dr Matthew J Seaman MA, D.Phil.                   7 Priory Courtyard
                                                  Flat 3
PGP: http://www.infracaninophile.co.uk/pgpkey     Ramsgate
                                                  Kent, CT11 9PW

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