Bash - Initialize variable to output of command "permission denied"

Question

I'm writing a small bash script to get the number out of each file name. For example the file name helloworld1.txt would produce 1.

When attempting to set the output to the variable i I get an error for each file.

line 5: 985.txt: Permission denied

If I just echo the command echo $f | tr -dc '[0-9]' rather than assign it to a variable, everything is good.

#!/bin/bash

for f in *
do
        i=`$f | tr -dc '[0-9]'` // Permission denied.
        echo $i
done

Answer 1

You have missed echo here. The line

i=`$f | tr -dc '[0-9]'`

should be

i=`echo $f | tr -dc '[0-9]'`

Answer 2

The line in the loop without a subshell, pipe and additional process per file:

i=${f//[^0-9]/}

The parameter substitution removes all non-digits.