Replacing environment variables in a properties file

sed 's/$HOSTNAME/myhost/g;s/$unconfigured//g' yourfile.txt > another_file.txt

Based on updates in your question, this won't be a good solution.

This is based on an answer to a related question. I've hacked at it (I'm unfamiliar with perl) to remove undefined vars.

perl -p -e 's/\$\{([^}]+)\}/defined $ENV{$1} ? $ENV{$1} : $&/eg; s/\$\{([^}]+)\}//eg' yourfile.txt

Should work for any input text file, however you will need to define vars using the ${...} format which simplifies the string matching.

(rant regarding the evilness of eval moved to a separate post so as not to confuse readers)

This is what envsubst is for.

echo 'Hello $USER'
Hello $USER
echo 'Hello $USER' | envsubst
Hello malvineous

You would probably use it more like this though:

envsubst < input.txt > output.txt

envsubst seems to be part of GNU gettext.

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