Bash - assign array into variable as string

(...) introduces a subshell. So $URL wouldn't be set after that (still have the value it had before the subshell). You want:

IFS=-
read -r -a host_name_array <<< "$(hostname)" 
unset 'host_name_array[${#host_name_array[@]}-1]'
URL="${host_name_array[*]}"

"${host_name_array[*]}" joins the elements on the arrays on the first character of $IFS just like "$*" does in standard sh.

If the reason why you're using a subshell is because you don't want to modify $IFS globally, you could do that in a function where you give $IFS a local scope:

f() {
  local IFS=-
  ...
}
f

Or use command substitution that also creates a subshell but allows passing data to the parent shell:

URL=$(IFS=-; printf '%s\n' "${host_name_array[*]}")

Here though I'd use standard sh expansion to remove that trailing component:

URL=$(uname -n) # uname -n is the standard equivalent of hostname
URL=${URL%-*}

It has several advantages over the above:

• it works even if the text contains newline characters (very unlikely for host names here though);
• if the text doesn't contain -, it doesn't remove anything;
• it is more efficient. read <<< $(hostname) means running hostname, read its output through a pipe, storing that in a temp file and have read read the first line of that;
• it doesn't clobber the variable namespace with those temporary variables
• it is shorter;
• it is portable. No need to install bash to run that code. The system's sh will suffice.

In any case, remember to quote your variables when using them in list contexts:

printf '%s\n' "$URL"

When doing:

echo $URL

You're invoking the split+glob operator. So, if $IFS still contains -, that $URL would be split on -, and echo would output those words separated by spaces.

You could do this by outputting the elements separated by -, and then strip off the last - by parameter expansion:

$ var=$(printf '%s-' "${host_name_array[@]}")

$ echo "$var"
foo-bar-spam-egg-

$ var="${var%-}"

$ echo "$var"
foo-bar-spam-egg

Also you need ${host_name_array[@]} instead of ${host_name_array[*]} to prevent outputting the elements as a whole separated by first character of IFS.

What you are trying to do can be achieved by a simple parameter expansion:

${var%-*}

Example:

$ var=$(hostname)

$ echo "${var%-*}"

bash specific answer

As this already use bashisms, sed, tr or any other external tool are useless:

IFS='-' read -r -a host_name_array < <(hostname)
#removing the last part of string after last "-"
unset host_name_array[${#host_name_array[@]}-1]

shorted=${host_name_array[*]}
echo ${shorted// /-}

Or simplier:

read myvar < <(hostname)
echo ${myvar%-*}