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That looks a bit garbled, this should be:
[ ! -z "$PS1" ]
Note the spaces around the brackets...

which is equivalent to
[ -n "$PS1" ]
or
[ "$PS1" ]
But the logical counterpart of [ ! -t 0 ]...

In addition to suggested checkscript the script, there are a couple more in the original script:
grep -En '==|\[\[|\[[0-9@]\]|=\(|\$\[|=~' script
80: if [ ! "x$curl" == "x" ];then
118: if [...