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

This solution allows you to keep the same formatting in the ouput file

Copy and paste the following lines in your script

cat $1 | while read line
do
  eval $line
  echo $line
  eval echo $line
done | uniq | grep -v '\$'

this will read the file passed as argument line by line, and then process to try and print each line twice: - once without substitution - once with substitution of the variables. then remove the duplicate lines then remove the lines containing visible variables ($)

Answer 2

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
....

Answer 3

Yes eval should be used carefully, but it provided me this simple oneliner for my problem. Below is an example using your filename:

eval "echo \"$(<Text_File.msh)\""

I use printf instead of echo for my own purposes, but that should do the trick. Thank you abyss.7 providing the link that solve my problem. Hope it helps.

Answer 4

If the variables you want to translate are known and limited in number, you can always do the translation yourself:

sed "s/\$LOG_FILE_PATH/$LOG_FILE_PATH/g" input > output

And also assuming the variable itself is already known