As the users says it is using **an ancient unix shell** , lets try another version of the script:
#!/bin/sh
#test version for ancient shell.
cd graham/quant
for file in *.biz
do
grep -q "max" "$file"
if [ "$?" = "0" ]; then
maximum.sh "$file"
fi
grep -q "milan" "$file"
if [ "$?" = "0" ]; then
milan.sh "$file"
fi
grep -q "min" "$file"
if [ "$?" = "0" ]; then
minimum.sh "$file"
fi
done
cd $OLDPWD