Variable Expansion when using variable within filename [duplicate] - bash

Question

This question already has answers here:
How to echo "$x_$y" in Bash script?
(4 answers)
When do we need curly braces around shell variables?
(7 answers)
Closed 3 years ago.
When using Bash I want to WGET multiple files from a server so I write a script with a For-loop that increments a counter to match the numbering of the files.
But I want to include the title of the file AND the number in which order the file appears (the "ID" of the file). So the the file has a URI of "example.com/files/hello_world.txt", with the ID of 42 and the title is "Hello World" when WGET it, the downloaded file should have the name "42_Hello_World.txt".
I tried the following code:
#! /bin/bash
# Init
index=42
title="Hello World"
# Replace blanks with underscore
title=${title/ /_}
# Concat fileName
fileName="$index_$title.txt"
echo $fileName
but the output is just "Hello_World.txt". When I change the order of $title and $index the output is "42.txt"
Can someone explain to me why this happens and how to solve it?
tl;dr
When using two or more variables in bash when evaluating a string only the last variable is "expanded". The first one is ignored. WHY???

Answer 1

_ is a valid character for an identifier, so $index_$title.txt is interpreted as the concatenation of two parameter expansions, $index_ and $title. To explicitly delimit the parameter name, use the full ${...} form:
fileName=${index}_$title.txt
The braces are not necessary for $title, because the following . cannot be interpreted as part of a parameter name (though the braces are certainly permitted: ${index}_${title}.txt).
Since index_ is not defined, $index_ expands to the empty string.

Answer 2

Yes. The explanation is the the _ character is a valid character in a variable name, so that your expressions are expanding the (undefined) variables $index_ and $title_ as empty strings. (The . is not a valid name character, so it terminates the 2nd name automatically.) Do this instead:
$ fileName="${index}_$title.txt"
$ echo $fileName
42_Hello_World.txt
$ echo "${title}_$index.txt"
Hello_World_42.txt

Answer 3

Could you please try following. This change should provide you your expected results. It is simple your variable "$index_$title.txt" is considered as you are concatenating 2 variables (index_ and title) so its better to quote _ like --> "_" and tell shell that it is a string.
index="42"
str="Hello World"
# Replace blanks with underscore
title=${str/ /_}
# Concat fileName
fileName=$index"_"$title".txt"
echo $fileName
In this nice url, you could see the last example of VALID variables(_ is there in the list):
https://bash.cyberciti.biz/guide/Rules_for_Naming_variable_name

Answer 4

The _ in the filename is not helping. _ is a valid variable character, and bash thinks that you want a variable called $index_ followed by $title, which isn't what you want. You can either:
Change the underscore character to an invalid variable name
Change to filename=$title"_"$index".txt" or
put brackets around $index
Hope this helps!
EDIT: You already have an answer here! How to echo "$x_$y" in Bash script?