grep 0: /proc/drbd | sed 's~.*cs:\([A-z]*\).*~\1~'

for arg in `cat /proc/drbd | grep '^ 0:'`; do
    case "$arg" in
        cs:*)
             STATE="${arg#cs:}"
             ;;
    esac
done
echo "$STATE"

# awk -F':' '$1 == " 0" {print $3}' /proc/drbd
Connected ro

# cat /proc/drbd |grep 0:| cut -d" " -f3
cs:Connected

awk -F':' '/0:/{split ($3,A," ");print A[1]}' /proc/drbd

# grep 0: /proc/drbd | sed 's~.*cs:\([A-z]*\).*~\1~'
sed: -e выражение #1, символ 23: Неверный конец диапазона

# awk -F':' '/0:/{split ($3,A," ");print A[1]}' /proc/drbd
Connected

# sed -r '/0:/{s/.*cs:([^ ]+) .*/\1/p};d' /proc/drbd
Connected

grep 0: /proc/drbd | sed 's~.*cs:\([A-Za-z]*\).*~\1~'

# echo '0: cs: Connected ro:Primary/Secondary ds:UpToDate/UpToDate C r----' | grep -Po '(?<=0: cs:).*?(?=:)
Connected ro

grep -Po '(?<=0: cs: ).*?(?=:ro)' /proc/drbd

x# grep 0: /proc/drbd | sed 's~.*cs:\([A-Za-z]*\).*~\1~'
Connected

# grep -Po '(?<=0: cs: ).*?(?=:ro)' /proc/drbd

grep -Po '(?<=0: cs: ).*?(?= ro:)' /proc/drbd