how to use variable variable names in bash script [duplicate]

Question

I'm trying to get the values of some php variables into a bash script in order to set other bash variables. Basically I need to be able to access the value of a variable - variable name.

the script can find & read the file, find the php variables, but when I try to set them it just hangs. Here is what I have:

variables=(database_user database_password dbase);

paths=(modx_core_path modx_connectors_path modx_manager_path modx_base_path);

for index in "${variables[@]}"; do
index=\$$index
echo $index;
$index="$(grep -oE '\$${!index} = .*;' $config | tail -1 | sed 's/$${!index} = //g;s/;//g')";
done

not sure what I am doing wrong here...

@123, please don't suggest the ABS as a reference; it's rather infamous for showcasing bad practices. BashFAQ #6 is a much more up-to-date and actively maintained discussion of indirect variables. — Charles Duffy
– Charles Duffy, Commented Feb 8, 2017 at 21:49

Fred · Accepted Answer · 2017-02-08 21:58:05Z

You are trying to perform an indirect assignment.

You should get rid of these two lines :

index=\$$index
echo $index;

By simply writing :

echo "${!index}"

Which does an indirect expansion cleanly (gives you the value of the variable whose name is contained in variable index).

Next, the problematic line is this one:

$index="$(grep -oE '\$${!index} = .*;' ... (rest omitted)

In Bash, an assignment cannot begin with a $.

One way you can perform an indirect assignment is this (after removing the index re-assignment as suggested above) :

printf -v "$index" "$(grep -oE '\$${!index} = .*;' ... (rest omitted)

This uses the -v option of printf, which causes the value passed as the final argument to be assigned to a variable, the name of which is passed to the -v option.

There are other ways of handling indirect assignment/expansions, some with code injection risks, as they use (potentially uncontrolled) data as code. This is something you may want to research further.

Please note I am assuming the actual grep command substitution works (I have not tested).

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