bash populate an array in loop

Question

How can i populate an array in loop? I'd like to do something like that:

declare -A results

results["a"]=1
results["b"]=2

while read data; do
results[$data]=1
done

for i in "${!results[@]}"
do
  echo "key  : $i"
  echo "value: ${results[$i]}"
done

But it seems that I cannot add anything to an array within for loop. Why?

Solution 1

What you have should work, assuming you have a version of Bash that supports associative arrays to begin with.

If I may take a wild guess . . . are you running something like this:

command_that_outputs_keys \
  | while read data; do
        results[$data]=1
    done

? That is — is your while loop part of a pipeline? If so, then that's the problem. You see, every command in a pipeline receives a copy of the shell's execution environment. So the while loop would be populating a copy of the results array, and when the while loop completes, that copy disappears.

Edited to add: If that is the problem, then as glenn jackman points out in a comment, you can fix it by using process substitution instead:

while read data; do
    results[$data]=1
done < <(command_that_outputs_keys)

That way, although command_that_outputs_keys will receive only a copy of the shell's execution environment (as before), the while loop will have the original, main environment, so it will be able to modify the original array.

Solution 2

That seems to be working fine:

$ cat mkt.sh 
declare -A results

results["a"]=1
results["b"]=2

while read data; do
  results[$data]=1
done << EOF
3
4
5
EOF

for i in "${!results[@]}"
do
  echo "key  : $i"
  echo "value: ${results[$i]}"
done

$ ./mkt.sh 
key  : a
value: 1
key  : b
value: 2
key  : 3
value: 1
key  : 4
value: 1
key  : 5
value: 1
$ 

Ubuntu 11.10 here, bash: GNU bash, version 4.2.10(1)-release (x86_64-pc-linux-gnu).