How to iterate through a variable while skipping one of the values?

Question

Using kernel 2.6.x

Note: In a previous question, I tried to accomplish this with a bash array. On this OS, non-sh shells are installed as Entware-NG packages and can't be used since they load after the script runs.

Goal: Script the creation of iptables rules using sh (not bash, zsh, etc.) with the following format. For each interface in $NAME ...

• Create a rule for it's source network IP address from the corresponding position in $NETID. The first value in $NAME has a source network address built from the first value in $NETID. For example, -i eth1 corresponds with -src 192.168.10.0/24 and -i eth2 corresponds with 192.168.20.0/24.
• Build the destination IP address for the values in $NETID not used in the source network address. For example, -src 192.168.10.0/24 -dst 192.168.20.0/24.
• Build each rule with one destination network address. Do not specify multiple addresses for the -dst value.

Desired Result:

iptables -I FORWARD -i eth1 -src 192.168.10.0/24 -dst 192.168.20.0/24
iptables -I FORWARD -i eth1 -src 192.168.10.0/24 -dst 192.168.30.0/24
iptables -I FORWARD -i eth2 -src 192.168.20.0/24 -dst 192.168.10.0/24
iptables -I FORWARD -i eth2 -src 192.168.20.0/24 -dst 192.168.30.0/24
iptables -I FORWARD -i eth3 -src 192.168.30.0/24 -dst 192.168.10.0/24
iptables -I FORWARD -i eth3 -src 192.168.30.0/24 -dst 192.168.20.0/24

Problem: The script below does not create a 1:1 relationship with the interface name and -src value. For example, eth1 and 192.168.10.0/24, eth2 and 192.168.20.0/24, etc.

---

#!/bin/sh
NETID="10 20 30"
NAME="eth1 eth2 eth3"



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

   for j in $NETID; do
      [ "$i" -eq "$j" ] && continue
      darg="192.168.$j.0/24"
      for k in $NAME; do
         echo "iptables -I FORWARD -i $k -s $sarg -d $darg -j DROP"
      done
   done
done

Answer 1

You main problem is that you have a 1:1 relationship between the interface names (eth1, eth2,...) and the associated networks (10.x, 20.x,...) but there is nothing in your code that explicitly establishes that 1:1 relationship.

You must somewhere bind ethN to 192.168.N.x. You can do it with a single NETS variable which would contain a list of interface:N pairs. See my code below:

#!/bin/sh
NETS="eth1:10 eth2:20 eth3:30"

for net in $NETS; do
    k="${net%:*}"          # Yields the name of the interface
    i="${net#*:}"          # Yields the number bound to the interface
    sarg="192.168.$i.0/24"

    for net in $NETS; do
        j="${net#*:}"      # Yields the number bound to the interface
        [ "$i" -eq "$j" ] && continue
        darg="192.168.$j.0/24"
        echo "iptables -I FORWARD -i $k -s $sarg -d $darg -j DROP"
    done
done

Notes:

• I removed the for k in $NAME loop entirely since it seems to contradict what you want to achieve.

• You could even put the network in the definition of NETS and read sarg and darg directly from it. Something like:

  NETS="eth1:192.168.10.0/24 eth2:192.168.20.0/24 eth3:192.168.30.0/24"
  

[update] Combined with my answer to your other question, this would give this script:

#!/bin/sh
NETID="10 20 30"
NAME="eth1 eth2 eth3"
NETS=$(set -- $NETID; for iface in $NAME; do echo "$iface:192.168.$1.0/24"; shift; done)

for net in $NETS; do
    iface="${net%:*}"
    sarg="${net#*:}"

    for net in $NETS; do
        darg="${net#*:}"
        [ "$sarg" != "$darg" ] && echo "iptables -I FORWARD -i $k -s $sarg -d $darg -j DROP"
    done
done

Answer 2

Using POSIX Awk, and not worrying about the sequence of the output lines:

echo | awk -v 'netids=10 20 30' -v 'names=eth1 eth2 eth3' '
  BEGIN {
    netidcount = split(netids, netid)
    namecount = split(names, name)
    if (netidcount != namecount) {
      print "Error: You must pass the same number of names and netids."
      exit 1
    }
    for (i in name) {
      for (j in name) {
        if (i == j) {
          continue
        }
        printf "iptables -I FORWARD -i %s -src 192.168.%s.0/24 -dst 192.168.%s.0/24 -j DROP\n", name[i], netid[i], netid[j]
      }
    }
  }'

Of course you can vary this; for instance you can put the awk script above in a file and call it with:

echo | awk -v ... -v ... -f script.awk

Or, you can put the netids and names in data.txt, perhaps formatted like so:

eth1 10
eth2 20
eth3 30

And write the Awk script file script.awk like so:

#!/bin/awk
NF != 2 {
  error = 1
  print "Error: Each line of input must contain one interface name and one number."
  exit
}
(($1 in nameseen) || ($2 in netidseen)) {
  error = 1
  print "Error: Each interface name and network id in input must be unique."
  exit
}
{
  name[NR] = $1
  netid[NR] = $2
  nameseen[$1]
  netidseen[$2]
}
END {
  if (error) {
    exit error
  }
  for (i in name) {
    for (j in name) {
      if (i == j) {
        continue
      }
      printf "iptables -I FORWARD -i %s -src 192.168.%s.0/24 -dst 192.168.%s.0/24 -j DROP\n", name[i], netid[i], netid[j]
    }
  }
}

And then call it like so:

awk -f script.awk data.txt

Lots of choices.

I would probably have data.txt look like:

eth1 192.168.10.0/24
eth2 192.168.20.0/24
eth3 192.168.30.0/24

Modifying the script to work with this data format is left as an exercise to the reader. ;)

---

_{(Note: if you use this code, please do remember to include a link to this answer as attribution.)}

Answer 3

POSIX sh version:

for n in 1 2 3; do
  src="$n"0
  interface="eth$n"
  for dest in 10 20 30; do
    [ "$dest" = "$src" ] && continue
    printf 'iptables -I FORWARD -i %s -src %s -dst %s -j DROP\n' "$interface" "192.168.$src.0/24" "192.168.$dest.0/24"
  done
done

I've made some assumptions about how you're tying interface to source for your rules. But this does produce the desired output you showed.