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Accepted
Rob Spieldenner
Asked: 2010-04-20 08:49:14 +0800 CST

How do I reference the value of a constructed environment variable in a loop?

  • 772

What I'm trying to do is loop over environment variables. I have a number of installs that change and each install has 3 IPs to push files to and run scripts on, and I want to automate this as much as possible (so that I only have to modify a file that I'll source with the environment variables). The following is a simplified version that once I figure out I can solve my problem.

So given in my.props:

COUNT=2
A_0=foo
B_0=bar
A_1=fizz
B_1=buzz

I want to fill in the for loop in the following script

#!/bin/bash
. <path>/my.props

for ((i=0; i < COUNT; i++))
do
  <script here>
done

So that I can get the values from the environment variables. Like the following(but that actually work):

echo $A_$i $B_$i

or

A=A_$i
B=B_$i
echo $A $B

returns foo bar then fizz buzz

linux bash

4 Answers

  1. Best Answer
    $ num=0
$ declare A_$num=42  # create A_0 and set it to 42
$ echo $A_0
42
$ b=A_$num           # create a variable with the contents "A_0"
$ echo ${!b}         # indirection
42

    You can iterate num and use declare to create the variables and indirection to reference them. But wouldn't you really prefer to use arrays?

    for i in foo bar baz
do
    some_array+=($i)    # concatenate values onto the array
done

    or

    string="foo bar baz"
some_array=($string)

    How about this?

    $ cat IPs
192.168.0.1
192.168.0.2
192.168.0.3
$ cat readips
#!/bin/bash
while read -r ip
do
    IP[count++]=ip                 # is it necessary to save them in an array,
    do_something_directly_with ip  # if you use them right away?
done < IPs
    • 2 
  2. A=`eval echo \$A_$i`
    • 1

  3. Found the answer at Indirect References Chapter of Advanced Bash Scripting Guide

    I needed to do

    eval A=\$A_$i
eval B=\$B_$i
    • 1

  4. Not sure I entirely follow your question, but if you want the variables availbe to children scripts use the export built-in:

    kbrandt@kbrandt:~/scrap$ help export
export: export [-nf] [name[=value] ...] or export -p
    NAMEs are marked for automatic export to the environment of
    subsequently executed commands.

    From what I think you might trying to do, I would just put the IPs in one file, one IP per line. Then have the install script read the ip as a command line argument:

    while read ip; do
   ./install-script "$ip"
done < file_with_ips

    If there is more than one piece of data per record, than you can deliminate them:

    kbrandt@kbrandt:~/scrap$ cat file_with_data 
192.168.2.5:foobar:baz
kbrandt@kbrandt:~/scrap$ while IFS=":" read -r ip data1 data2; do 
> echo "$ip@$data1@$data2"
> done < file_with_data
192.168.2.5@foobar@baz
    • 0