Oi eu usaria algo como:
#!/bin/bash
if [[ 'file -b $1' == "ASCII text" ]] ; then
echo "file contains ascii characters only"
else
echo "file contains non-ascii characters"
fi
Parece:
$ filetest.sh 1.txt
file contains ascii characters only
$ filetest.sh PHOTOS/HPIM0532.jpg
file contains non-ascii characters