在 bash 中迭代 2 个数组 - 仅循环每个数组中的第一个值

Question

So i am trying to loop over two array's in bash but im struggling to get this to work, it seems to stop on the first iteration and doesn't loop the entire array, please note i am using IFS as i want to define the separator.

#!/bin/bash

protocol="udp,tcp-client,udp,"
port="1111,2222,3333,"

# split comma separated string into list from VPN_REMOTE_PROTCOL variable
IFS=',' read -a vpn_remote_protocol <<< "${protocol}"

# split comma separated string into list from VPN_REMOTE_PORT variable
IFS=',' read -a vpn_remote_port <<< "${port}"

for index in "${!vpn_remote[*]}"; do

        echo "iptables -A OUTPUT -o docker_int -p ${vpn_remote_protocol[$index]} --dport ${vpn_remote_port[$index]} -j ACCEPT"

done

output is:-

iptables -A OUTPUT -o docker_int -p udp --dport 1111 -j ACCEPT

i want the output to be:-

iptables -A OUTPUT -o docker_int -p udp --dport 1111 -j ACCEPT
iptables -A OUTPUT -o docker_int -p tcp-client --dport 2222 -j ACCEPT
iptables -A OUTPUT -o docker_int -p udp --dport 3333 -j ACCEPT

Answer 1

The most immediate problem in the original code was use of vpn_remote as the name of the array for which we're searching for keys, instead of vpn_remote_protocol or vpn_remote_port . The next problem is use of "${!array[*]}" , which generates a single string concatenating all the keys together ; just as for item in "two words" only runs once , with two words as the value assigned to item, looping over "${!vpn_remote_port[*]}" here would be running for index in... only once , with "0 1 2" as the item. (By contrast, with the incorrect variable name, "" is what "${!array[@]}" evaluates to; in a numeric context that's treated as 0 , which is why you see the first item and no others).

---

Moral Of This Story

Use "${!array[@]}" , never "${!array[*]}" , ${!array[*]} or ${!array[@]} without understanding the differences and having an explicit reason.

Thus:

#!/bin/bash

protocol="udp,tcp-client,udp,"
port="1111,2222,3333,"

IFS=',' read -a vpn_remote_protocol <<< "${protocol}"
IFS=',' read -a vpn_remote_port <<< "${port}"

for index in "${!vpn_remote_port[@]}"; do
    echo "iptables -A OUTPUT -o docker_int -p ${vpn_remote_protocol[$index]} --dport ${vpn_remote_port[$index]} -j ACCEPT"
done

...which, when run in an online interpreter , emits as output:

iptables -A OUTPUT -o docker_int -p udp --dport 1111 -j ACCEPT
iptables -A OUTPUT -o docker_int -p tcp-client --dport 2222 -j ACCEPT
iptables -A OUTPUT -o docker_int -p udp --dport 3333 -j ACCEPT

---

Going Deeper: Why The Other Alternatives Are Each Wrong

Why "${!array[@]}" ? Let's compare behavior of each, but in a more challenging case when our keys aren't purely numeric (though we could generate bugs by using the incorrect approaches even with purely-numeric keys, if we had numbers in our IFS value):

declare -A array=(
  ["first key"]="first value"
  ["second key"]="second value"
  ["third key"]="third value"
)

printf '%s\n' 'Using "${!array[@]}":'
printf ' - %s\n' "${!array[@]}"

printf '\n%s\n' 'Using "${!array[*]}":'
printf ' - %s\n' "${!array[*]}"

printf '\n%s\n' 'Using ${!array[*]}:'
printf ' - %s\n' ${!array[*]}

printf '\n%s\n' 'Using ${!array[@]}:'
printf ' - %s\n' ${!array[@]}

...for which we see the following output :

Using "${!array[@]}":
 - second key
 - third key
 - first key

Using "${!array[*]}":
 - second key third key first key

Using ${!array[*]}:
 - second
 - key
 - third
 - key
 - first
 - key

Using ${!array[@]}:
 - second
 - key
 - third
 - key
 - first
 - key

So:

• "${!arrayname[*]}" is wrong because it expands to a single string with all the keys concatenated (with the first character in IFS -- a space by default -- between them).
• ${!arrayname[*]} and ${!arrayname[@]} are both wrong because the keys are subject to word-splitting and glob expansion before they can be treated as a list of items
• "${!arrayname[@]}" does the Right Thing, treating each key as a single word, not subject to operations that could mangle it first.

Answer 2

You should iterate over ${!vpn_remote_port[*]} or ${!vpn_remote_protocol[*]} (without ")

But what exactly is happening here? Why only looping over first value in each array? The main reason is wrong array name so index var is empty and it's only one loop. Because you are looping over this "" empty string. And if we try to fetch data from ${vpn_remote_port[$index]} with empty $index it'll return first value(index 0).