Question

I have a text file that contains references to variables and lets a user set up the formatting they want around variables, say something like

 The date is $DATE
 The time is $TIME

I then want to read this text file in, replace the variables, and print the result to stdout using a bash script. The closest thing I’ve gotten is using “echo” to output it,

 DATE="1/1/2010"
 TIME="12:00"
 TMP=`cat file.txt`
 echo $TMP

However, the output ends up all on one line, and I don’t want to have \n at the end of every line in the text file. I tried using “cat << $TMP”, but then there are no newlines and the variables inside the text aren’t getting replaced with values.

Answer 1

You can use eval to ensure that variables are expanded in your data file:

DATE="1/1/2010"
TIME="12:00"
while read line
do
  eval echo ${line}
done < file.txt

Note that this allows the user with control over the file content to execute arbitrary commands.

If the input file is outside your control, a much safer solution would be:

DATE="1/1/2010"
TIME="12:00"
sed -e "s#\$DATE#$DATE#" -e "s#\$TIME#$TIME#" < file.txt

It assumes that neither $DATE nor $TIME contain the # character and that no other variables should be expanded.