shell_exec not if does not work

check.sh has got echo "error01" hope you can help this does not work

<?php
$c = shell_exec("check.sh 2>&1");
if($a == "error01"){exit("TEST");}
echo "Wellcome";
?>

But this does

<?php
$c = shell_exec("check.sh 2>&1");
if(isset($c)){exit("TEST");}
echo "Wellcome";
?>