How to substitute shell variables in complex text files

Question

I have several text files in which I have introduced shell variables ($VAR1 or $VAR2 for instance).

I would like to take those files (one by one) and save them in new files where all variables would have been replaced.

To do this, I used the following shell script (found on StackOverflow):

while read line
do
    eval echo "$line" >> destination.txt
done < "source.txt"

This works very well on very basic files.

But on more complex files, the "eval" command does too much:

• Lines starting with "#" are skipped

• XML files parsing results in tons of errors

Is there a better way to do it? (in shell script... I know this is easily done with Ant for instance)

Kind regards

Answer 1

Looking, it turns out on my system there is an envsubst command which is part of the gettext-base package.

So, this makes it easy:

envsubst < "source.txt" > "destination.txt"

Note if you want to use the same file for both, you'll have to use something like moreutil's sponge, as suggested by Johnny Utahh: envsubst < "source.txt" | sponge "source.txt". (Because the shell redirect will otherwise empty the file before its read.)