O GRAPHEND
deve estar em uma nova linha.
#!/bin/bash
graph=$(cat <<GRAPHEND
graph match {
node[style=filled shape=point label= ""];
size="40.0,40.0";
fontsize=10.0;
overlap=false ;
spline=true;
nodesep=4.0;
"aaa" -- "aab" [penwidth=2.25 color="red" label="4" fontsize=7.0];
}
GRAPHEND
)
echo $graph