How to iterate through a bash array while removing an element?

Question

GNU bash, version 4.3.42(1)-release (arm-openwrt-linux-gnu)

I'm trying to script an iptables rule using a bash array.

$NETID is used to create network addresses. For example, 192.168.10.0/24, 192.168.20.0/24, etc.

I'm trying to create a temporary variable ($NETID_TMP) where the "i" in ${NETID[@]} is removed from $NETID_TMP. For example, if "i" = 10, NETID_TMP=(20 30 40).

Script:

    #!/opt/bin/bash
    NETID=(10 20 30 40)
    for (( i=0; i<${#NETID[@]}; i++ )); do
       NETID_TMP=(${NETID[*]})
       unset NETID_TMP[${NETID[i]}]
       iptables -I FORWARD -s 192.168.${NETID[i]}.0/24 -d 192.168.${NETID_TMP[0]}.0/24,192.168.${NETID_TMP[1]}.0/24,192.168.${NETID_TMP[2]}.0/24 -j DROP
    done

The result should be ...

iptables -I FORWARD -s 192.168.10.0/24 -d 192.168.20.0/24,192.168.30.0/24,192.168.40.0/24 -j DROP
iptables -I FORWARD -s 192.168.20.0/24 -d 192.168.10.0/24,192.168.30.0/24,192.168.40.0/24 -j DROP
iptables -I FORWARD -s 192.168.30.0/24 -d 192.168.10.0/24,192.168.20.0/24,192.168.40.0/24 -j DROP
iptables -I FORWARD -s 192.168.40.0/24 -d 192.168.10.0/24,192.168.20.0/24,192.168.30.0/24 -j DROP

Answer 1

netid=(10 20 30 40)

for i in "${netid[@]}"; do
    out="iptables -I FORWARD -s 192.168.$i.0/24 -d "

    for j in "${netid[@]}"; do
        (( i == j )) && continue
        out+="192.168.$j.0/24,"
    done
    out="${out%,} -j DROP"

    printf '%s\n' "$out"
done

This Bash script produces

iptables -I FORWARD -s 192.168.10.0/24 -d 192.168.20.0/24,192.168.30.0/24,192.168.40.0/24 -j DROP
iptables -I FORWARD -s 192.168.20.0/24 -d 192.168.10.0/24,192.168.30.0/24,192.168.40.0/24 -j DROP
iptables -I FORWARD -s 192.168.30.0/24 -d 192.168.10.0/24,192.168.20.0/24,192.168.40.0/24 -j DROP
iptables -I FORWARD -s 192.168.40.0/24 -d 192.168.10.0/24,192.168.20.0/24,192.168.30.0/24 -j DROP

through a double loop. The inner loop skips the iterations where the loop variable is numerically equal to the loop variable of the outer loop to create the desired effect.

Once you have the string in $out, you may eval it.

Alternatively, to avoid eval:

netid=(10 20 30 40)

for i in "${netid[@]}"; do
    sarg="192.168.$i.0/24"
    darg=""

    for j in "${netid[@]}"; do
        (( i == j )) && continue
        darg+="192.168.$j.0/24,"
    done
    darg="${darg%,}"

    iptables -I FORWARD -s "$sarg" -d "$darg" -j DROP
done

The variable substitution ${parameter%word} will remove word from the very end of $parameter, so darg="${darg%,}" will remove the comma at the end af $darg.

---

Update after question in comment:

The only thing bash-specific about the above is the use of the array $netid, the += operator to append to the $darg string and ((...)). We may turn this into a sh script like this (here assuming $IFS still has its default value):

netid="10 20 30 40"

for i in $netid; do
    sarg="192.168.$i.0/24"
    darg=""

    for j in $netid; do
        [ "$i" -eq "$j" ] && continue
        darg="${darg}192.168.$j.0/24,"
    done
    darg="${darg%,}"

    iptables -I FORWARD -s "$sarg" -d "$darg" -j DROP
done

If you have a separate list of numbers that you'd like to plug in for $darg, use that in the inner loop in place of $netid. It should be a string of space-separated numbers.

Answer 2

It's easier with zsh:

#! /bin/zsh -
nets=(192.168.{10,20,30,40}.0/24)
for net ($nets) iptables -I FORWARD -s $net -d ${(j:,:)nets:#$net} -j DROP