Newbie's question about 'export'

1. Variable 'export' question.

Hi Everyone,

Quick question.... I am just curious to find out what does the
following syntax do? I came across this in a script that I was looking
at:

DATAPATH=${DATAPATH:-.,/api/scripts,/apps/misc}; export DATAPATH

When I do a echo $DATAPATH, the o/p looks like:
.,/api/scripts,/apps/misc

However, next if I do:
DATAPATH=${DATAPATH:-/home/test}; export DATAPATH
the o/p of the echo statement is still the same as above!

Normally, I would do it the following way, since I have been used to
this syntax:

DATAPATH=$DATAPATH:.:/api/scripts:/apps/misc; export DATAPATH
DATAPATH=$DATAPATH:/home/test; export $DATAPATH

the o/p looks like (which is correct & which is what I want):
.:/api/scripts:/apps/misc:/home/test

TIA,
Suhas

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