Simple and standard way to generate files based on templates?

Question

For my projects I need a way to generate some files based on template ones. I envision it this way:

• a template file contains data with something like {{var1}} in place of the variables to be replaced

• other file contains a list of key = value values

• after applying a tool, a new file with concrete values should be generated

Instead of the 2nd file, a list of key = value pairs could be provided via the command line or, which is the same thing, some keys may require environmental variables instead of the keys-value in the 2nd file.

Are there standard and well known utilities used in Linux/Unix world that can do that?

Answer 1

It really is quite easy to script something like this. For instance, you can source the file with key=value pairs and then use that to replace the contents of the template. For example:

$ cat template
Hello $name,
someone told me you really like $thing!

$ cat values
name=Bob
thing=chocolate

Your script would then be:

#!/bin/sh

## source the key/value pairs
. "$1"

## Replace them in the template
sed -e "s/\\\$name/$name/g" -e "s/\\\$thing/$thing/g" "$2"

And you run it like this:

$ foo.sh /full/path/to/values template
Hello Bob,
someone told me you really like chocolate!

Of course, that requires you to know the names of the keys in advance. An alternative would be:

#!/bin/sh

tmpFile=$(mktemp)
cp -- "$2" "$tmpFile"

while IFS='=' read -r key value; do
  sed -i -e "s/\$$key/$value/g" "$tmpFile"
done < "$1"

cat "$tmpFile"

Another option would be to use eval to expand the variables. However, this is dangerous since it opens you up to code injection if the values in the file can be dangerous. If that's not an issue for you, try:

#!/bin/sh

## source the key/value pairs
. "$1"

## load the contents of the template
template="$(cat "$2")"
## Now print, but expanding the variables. We need to 
## unset IFS to protect the newlines.
IFS=
eval echo \"$template\"