Это должно сделать это правильно в bash:
#!/bin/bash
#setting language variables for subshell making sure we grep for the right word
LC_ALL=C
LANG=C
# retrieve IP from user input
read -p "Input your network (example: 192.168.0): " my_net
for i in $(seq 1 254);
do
ip="$my_net.$i"
check="$(ping -c1 "$ip")"
if [ "$(grep "Unreachable" <<<"$check")" != "" ]
then
echo "$ip is unreachable"
fi
done