[Wylug-help] Setting Default Variables in Shell Scripts

[Smylers]

Hi.  I've got a (Bourne) shell script which contains something along the
lines of:

  : ${NAME=Seymour}

When run (on Bash) this seems to be doing the same as[*0]:

  if [ -z $NAME ]
  then
    NAME=Seymour
  fi

Where is this documented?  In bash(1) searching for 'Assign Default
Values' suggests that this variant would work:

  : ${NAME:=Seymour}

-- and indeed it does.  But how come it also works without the (second)
colon?

The script in question runs /bin/sh, not Bash specifically, so I'm
presuming this is an old Bourne feature and not a Bash-ism.

Thanks.

Smylers

[*0]  Or, for anybody who knows Perl, it's the Shell equivalent of the
Perl idiom:

  $name ||= 'Seymour';