If you can conform to a bit of naming convention, one way to easily render template files in Bash is to have the template and bash variables names match. Then you can either use sed or envsubst to do the substitution.
For example, here are two variables being defined and then placeholders embedded into the template.
# define and export variables export first="1" export animal="dog" # define template string, heredoc without var evaluation read -r -d '' mytemplate <<'EOF' this is $first the $animal goes woof EOF
You can use use the envsubst utility to easily render the template.
$ echo "$mytemplate" | envsubst '$first $animal' this is 1 the dog goes woof
Or you can generate, then apply a list of sed substitutions.
# create list of sed replacements
sedcmd=''
for var in first animal ; do
printf -v sc 's/$%s/%s/;' $var "${!var//\//\\/}"
sedcmd+="$sc"
done
# do all template substitutions
sed -e "$sedcmd" <(echo "$mytemplate")
See my test_generic_variables_template.sh on github for a full example.
REFERENCES
howtobuildsoftware.com, replace bash variables in template file
stackoverflow, heredoc without variable evaluation
gnu.org, process substitution allowing process to be file descriptor
NOTES
variables do not have to be exported, they can also be set directly
$ echo "$mytemplate" | first=1 animal=dog envsubst '$first $animal'