awk command does not work properly when assigning output to variable

Question

I'm trying to convert space to underscore in a file name, my script is like below.

old_file=/home/somedir/otherdir/foobar 20170919.csv
new_file="$(basename "$old_file")" | awk 'gsub(" ","_")'

This script works fine when I use with echo command,

echo "$(basename "$old_file")" | awk 'gsub(" ","_")'

but when it comes to assigning the output to variables, it doesn't work...

Does anybody know the idea?

Answer 1

Actually no need of awk, please note below one replaces all space to underscore, not just filename, it can be path too

$ old_file="/home/somedir/otherdir/foobar 20170919.csv"
$ newfile="${old_file// /_}"
$ echo "$newfile"
/home/somedir/otherdir/foobar_20170919.csv