Create a dynamic string variable from a configuration text file in bash script

Question

I want to create a dynamic string variable that holds file extension from a configuration text file like this. The bad thing is that the string value contains regex expression:

EXCLUDE_EXTENSION="\.(log|txt|png)$" 

where the log, txt and png extensions I get it from a text file called excluded_ext.txt. So the content of the excluded_ext.txt is:

log txt png 

so whenever I add another extension in excluded_ext.txt, I will get an updated extension inside the variable EXCLUDE_EXTENSION. Example if I add an extra extension of 'log' inside excluded_ext.txt

log txt png log 

then the value of variable EXCLUDE_EXTENSION should be updated automatically to:

EXCLUDE_EXTENSION="\.(log|txt|png|log)$" 

I think I might need to use regex but I'm not sure how to achieve this.

#!/bin/sh # read from a text file EXCLUDED_TEXT=`cat excluded_ext.txt` # create array from the text file # Im not sure how to go next. 

Answer 1

Using paste and a command substitution:

EXCLUDE_EXTENSION='\.('"$(paste -d'|' -s excluded_ext.txt)"')$' 

Answer 2

If you're using bash (as per your title) rather than sh (as shown in your code snippet), then you should be able to do something like this:

mapfile -t exts < excluded_ext.txt # this creates the array OldIFS="$IFS" IFS='|' printf -v exclude_extension '\.(%s)$' "${exts[*]}" IFS="$OldIFS" echo "$exclude_extension" 

"${exts[*]}" expands to the elements of exts, separated by the first character of the current IFS.