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/lo         


        

Answer 1

Create an ascii file test.txt with the following content:

Try to replace this ${myTestVariable1} 
bla bla
....

Now create a file “sub.sed” containing variable names, eg

's,${myTestVariable1},'"${myTestVariable1}"',g;
s,${myTestVariable2},'"${myTestVariable2}"',g;
s,${myTestVariable3},'"${myTestVariable3}"',g;
s,${myTestVariable4},'"${myTestVariable4}"',g'

Open a terminal move to the folder containing test.txt and sub.sed.
Define the value of the varible to be replaced

myTestVariable1=SomeNewText

Now call sed to replace that variable

sed "$(eval echo $(cat sub.sed))" test.txt > test2.txt

The output will be

$cat test2.txt

Try to replace this SomeNewText 
bla bla
....