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I would like to make a bash script that will take 3 different variables and return just one based on the setting of another. So if 

TESTA_HOST=test1.google.com
TESTB_HOST=test2.google.com
TESTC_HOST=test3.google.com
M_INDEX=2

We want to search for all env variables that end in _HOST make an array that we can then use to compare to an index.

HOSTS="$(compgen -A variable | grep _HOST)"
export MAIN_URI="${HOSTS[M_INDEX]}"
echo $MAIN_URI

Expected output would be test3.google.com this doesn't work though since it is just returning the variable name and not the variable contents. What am I missing here?

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This,

HOSTS="$(compgen -A variable | grep _HOST)"

sets HOSTS to the single string

TESTA_HOST[newline]TESTB_HOST[newline]TESTC_HOST

Indexing it isn't very useful, ${HOSTS[0]} is the same as ${HOSTS}, and ${HOSTS[1]} etc. will be empty.

If, instead, you assign

HOSTS=( $(compgen -A variable | grep _HOST) )

Then output of compgen will be word-split (and expanded for globs), and HOSTS becomes an array containing the results. The values of that array are the names of the variables, you'll need an indirect reference to get their values. This would now expand to the value of the variable "pointed to" by index:

"${!HOSTS[M_INDEX]}"

Note that array assignment assumes you have the default value of IFS, and that the variables cannot have any whitespace or glob characters within them.

Note that if your variables were named HOST_this and HOST_that instead (with the fixed part in front), you could generate a list of their names with "${!HOST_@}", and assign that directly to an array, without involving compgen, grep, or word-splitting.

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