Re: [Help-bash] Adding missing bash features for safely re-usable bash code

[Eric Blake]

On 04/08/2013 12:10 PM, Greg Wooledge wrote:
> However, my personal preference for functions returning values (and this
> is not limited to strings; the same applies to integers larger than 255 or
> smaller than 0, or floating point values, or arrays) is to hard-code
> the variable "r" in both the caller and the callee:
> 
> rand() {
>   local max=$((32768 / $1 * $1))
>   while (( (r=$RANDOM) >= max )); do :; done
>   r=$(( r % $1 ))
> }
> 
> caller() {
>   local r
>   rand 17
>   if ((r < 10)); then ....
>   fi
> }
> 
> This gives the callee a place to store its return value, without requiring
> a subshell, and without requiring the re-parsing of an ASCII stream.
> If "r" is declared local in the caller, then its scope is limited to the
> caller and the callee.  Otherwise, it's global.  I get to choose.
> 
> Of course this means I can't use the variable "r" for any other purpose.
> Not that I was going to.  If you don't like that, you can use "___r" or
> whatever.

A slight variation on that theme is to have the caller tell the function
where to store the result:

rand() {
  local max=$((32768 / $2 * $2))
  local val
  while (( (val=$RANDOM) >= max )); do :; done
  val=$(( val % $2 ))
  eval $1=$val
}

caller() {
  local r
  rand r 17
  if ((r < 10)); then ...
  fi
}

Not shown is that you now have the burden of ensuring that the caller
doesn't pass in garbage for $1 which would be a disaster inside the
eval; but with the approach of telling a function where to put its
result, you no longer have to worry about choosing a naming convention
that the caller must follow for its called functions to work correctly.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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