How to expand shell variables in a text file?

Question

Consider a ASCII text file (lets say it contains code of a non-shell scripting language):

Text_File.msh:

spool on to '$LOG_FILE_PATH/logfile.log';
login 'username' 'password';
....

Now if this were a shell script I could run it as $ sh Text_File.msh and the shell would automatically expand the variables. What I want to do is have shell expand these variables and then create a new file as Text_File_expanded.msh as follows:

Text_File_expanded.msh:

spool on to '/expanded/path/of/the/log/file/../logfile.log';
login 'username' 'password';
....

---

Consider:

$ a=123
$ echo "$a"
123

So technically this should do the trick:

$ echo "`cat Text_File.msh`" > Text_File_expanded.msh

...but it doesn't work as expected and the output-file while is identical to the source.

So I am unsure how to achieve this.. My goal is make it easier to maintain the directory paths embedded within my non-shell scripts. These scripts cannot contain any UNIX code as it is not compiled by the UNIX shell.

Answer 1

This question has been asked in another thread, and this is the best answer IMO:

export LOG_FILE_PATH=/expanded/path/of/the/log/file/../logfile.log
cat Text_File.msh | envsubst > Text_File_expanded.msh

if on Mac, install gettext first: brew install gettext

see: Forcing bash to expand variables in a string loaded from a file