Create new file from templates with bash script

Template module for bash? Use sed, Luke! Here is an example of one of millions of possible ways of doing this:

$ cat template.txt 
#!/bin/sh

echo Hello, I am a server running from %DIR% and listening for connection at %HOST% on port %PORT% and my configuration file is %DIR%/server.conf

$ cat create.sh 
#!/bin/sh

sed -e "s;%PORT%;$1;g" -e "s;%HOST%;$2;g" -e "s;%DIR%;$3;g" template.txt > script.sh

$ bash ./create.sh 1986 example.com /tmp
$ bash ./script.sh 
Hello, I am a server running from /tmp and listening for connection at example.com on port 1986 and my configuration file is /tmp/server.conf
$ 

For simple file generation, basically doing

 . "${config_file}"
 template_str=$(cat "${template_file}")
 eval "echo \"${template_str}\""

would suffice.

Here ${config_file} contains the configuration variables in shell parseable format, and ${template_file} is the template file that looks like shell here document. The first line sources in the file ${config_file}, the second line puts the contents of the file ${template_file} into the shell variable template_str. Finally in the third line we build the shell command echo "${template_str}" (where the double quoted expression "${template_str}" is expanded) and evaluate it.

For an example of the contents of those two files, please refer to https://serverfault.com/a/699377/120756.

There are limitations what you can have in the template file or you need to perform shell escaping. Also if the template file is externally produced, then for security reasons you need to consider implementing a proper filtering prior to execution so that you will not for example lose your files when somebody injects the famous $(rm -rf /) in the template file.

You can do this using a heredoc. e.g.

generate.sh:

#!/bin/sh

#define parameters which are passed in.
PORT=$1
DOMAIN=$2

#define the template.
cat  << EOF
This is my template.
Port is $PORT
Domain is $DOMAIN
EOF

Output:

$ generate.sh 8080 domain.example

This is my template.
Port is 8080
Domain is domain.example

or save it to a file:

$ generate.sh 8080 domain.example > result

you can do this directly in bash, you do not even need sed. Write a script like that:

#!/bin/bash

cat <<END
this is a template
with $foo
and $bar
END

then call it like so:

foo=FOO bar=BAR ./template