Re: Another mapfile question.

[Steven W. Orr]

On 8/5/2011 9:08 AM, Maarten Billemont wrote:
> > IFS=
> > aa=()
> > while read line
> > do
> >   aa+=("$line")
> > done<  fn
>
> You should really put that IFS= on your 'read', so as not to break the default 
> wordsplitting for the rest of your script:

For purposes of giving concise examples on this email list, I wrote it as 
above. In practice, something like the above construct that modifies IFS would 
be responsible for restoring IFS. e.g.,

old_IFS="$IFS"
# do stuff
IFS="$old_IFS"

More often, the code would be placed in a subroutine where IFS is declared as 
a local variable. Within that routine, if IFS needs to be restored, then 
old_IFS would also be locally declared. If IFS did not need to be restored 
(within the routine) then old_IFS might not even need to be declared.

One trick that bit me a while back, is that the order of declaring IFS and 
old_IFS is important. It has to be done in this order:

foo()
{
    typeset -r old_IFS="$IFS" # Must come first
    typeset IFS

    stuff...
}

This is because the initial value to old_IFS is referring to the outer scoped 
IFS. It is nice though that the locally declared IFS gets an initial value 
from the outer scope :-)

> while IFS= read -r line
>
> > vs.
> > IFS=$'\n'
> > aa=($(<  fn))
>
> Don't leave expansions unquoted!  you're still permitting pathname expansion 
> here.
>
> IFS=$'\n' read -rd '' -a aa<  <(fn)
>
> > vs.
> > mapfile -t aa<  fn
>
> When compared to the above read, this will also read empty lines.  The read 
> operation will merge multiple newlines into a single delimiter (as it does for 
> all types of whitespace in IFS).
>
> The while read works fine but is verbose.  The mapfile is a bit more concise.  
> The read -a is fine and concise so long as you can live with the lack of empty 
> lines.  I doubt mapfile is much use to you that while read doesn't already give 
> you.


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