Loop Through Variables

Question

I am writing a bash script to use rsync and update files on about 20 different servers.

I have the rsync part figured out. What I'm having trouble with is going through a list of variables.

My script thus far look like this:

#!/bin/bash
SERVER1="192.xxx.xxx.2"
SERVER2="192.xxx.xxx.3"
SERVER3="192.xxx.xxx.4"
SERVER4="192.xxx.xxx.5"
SERVER5="192.xxx.xxx.6"
SERVER6="192.xxx.xxx.7"

for ((i=1; i<7; i++))
do
    echo [Server IP Address]
done

Where [Server IP Address] should be the value of the associated variable. So when i = 1 I should echo the value of $SERVER1.

I've tried several iterations of this including

echo "$SERVER$i"    # printed the value of i
echo "SERVER$i"     # printer "SERVER" plus the value of i ex: SERVER 1 where i = 1
echo $("SERVER$i")  # produced an error SERVER1: command not found where i = 1
echo $$SERVER$i     # printed a four digit number followed by "SERVER" plus the value of i
echo \$$SERVER$i    # printed "$" plus the value of i

It has been a long time since I scripted so I know I am missing something. Plus I'm sure I'm mixing in what I could do using C#, which I've used for the past 11 years.

Is what I'm trying to do even possible? Or should I be putting these values in an array and looping through the array? I need to this same thing for production IP addresses as well as location names.

This is all in an effort to not have to repeat a block of code I will be using to sync the files on the remote server.

Answer 1

Use an array.

#! /bin/bash
servers=( 192.xxx.xxx.2 192.xxx.xxx.3
          192.xxx.xxx.4 192.xxx.xxx.5
          192.xxx.xxx.6 192.xxx.xxx.7
)

for server in "${servers[@]}" ; do
    echo "$server"
done

Answer 2

As the other answers point out, an array is the most convenient way to do this. However, for completeness, the exact thing you are asking for is indirect expansion. Rewritten as follows, your sample will also work using this method:

#!/bin/bash
SERVER1="192.xxx.xxx.2"
SERVER2="192.xxx.xxx.3"
SERVER3="192.xxx.xxx.4"
SERVER4="192.xxx.xxx.5"
SERVER5="192.xxx.xxx.6"
SERVER6="192.xxx.xxx.7"

for ((i=1; i<7; i++))
do
    servervar="SERVER$i"
    echo "${!servervar}"
done

---

If you're OK with just putting the list of IP addresses in the the for loop, then you might also consider simply using some brace expansions to iterate over whatever you need:

#!/bin/bash

for server in \
192.xxx.xxx.{2..7} \
192.yyy.yyy.{42..50} \
192.zzz.zzz.254
do
    echo "$server"
done

But if you need to reuse the (possibly brace-expanded) list, then using the list to initialize an array would be the way to go:

#!/bin/bash

servers=(
192.xxx.xxx.{2..7} 
192.yyy.yyy.{42..50}
192.zzz.zzz.254 )

for server in "${servers[@]}"
do
    echo "$server"
done

Answer 3

While I would probably go with one of the array answers myself, I’d like to point out that it is possible to loop over names directly. You can do

for name in "${!SERVER*}"; do
    echo "${!name}"
done

or, in 4.3 and up, you can use a nameref:

declare -n name
for name in "${!SERVER*}"; do
    echo "$name"
done

Hat tip ilkkachu for the < 4.3 solution.

Answer 4

Everyone who said you should use an array is correct, but -- as an academic exercise -- if you were determined to do it with separate variables ending in a number (SERVER1, SERVER2, etc.), this is how you'd do it:

for ((i=1; i<7; i++))
do
    eval echo \"\$SERVER$i\"
done