Generating content stored under a dynamic variable in Shell Script

Question

I am stuck with the below code and any help would be greatly appreciated:

I am trying to assign contents inside a dynamically generated variable to another variable. But instead of the value of the variable getting assigned, I could see that the variable name is getting assigned. I tried (echo/eval/!) but nothing is working. Could someone please help me with the solution?

Objective: To assign value 24 (saved under variable msd_ext_len) to final_count variable.

Variable Declaration

iter_val=1

var_1="msd_ext"

msd_ext_len=24

Code:

eval "final_count"=\${var_${iter_val}}_len

Output --> msd_ext_len

Expected Output --> 24

If I add one more line of code as shown below along with above code and I am getting the output:

count_val=${!final_count}

But can these 2 lines of code be clubbed into a single line of code?

Answer 1

So, first, every time you do something like ${var_${iter_val}}, i.e. trying to append a number to a variable name, you should be using an array instead:

arr=(a b c)
arr[1]=x
i=1
echo "${arr[i]}"

If you need to have something indexed by strings instead, use an associative array (declare -A arrayname in Bash).

The combination of those, say having distinct properties on some "objects" in an array, is harder. You could use two separate arrays and then use foo[i] and bar[i].

If you still need to indirectly point to those arrays by name, you need a name reference.

foo=(a b c)
bar=(x y z)
declare -n arrname=foo
echo "${arrname[i]}"

I can't tell from your code what your data structure is supposed to be, exactly, so it's hard to give an answer for just that scenario.

---

Trying to rewrite your example, you could do something like this:

varnames=()
varnames[1]=msd_ext
varnum=1
msd_ext_len=24

declare -n srcvar="${varnames[varnum]}_len"
declare -n dstvar=final_count
dstvar=$srcvar

echo $final_count

That still requires using an array for varnames, since the nested expansion ${var_$foo} doesn't really work.

See:

• Does bash provide support for using pointers?
• the page Arrays on BashGuide (associative arrays at the bottom)