Templating with Linux in a Shell Script?

what I want to acomplish is:

1.) Having a config file as template, with variables like $version $path (for example apache config)

2.) Having a shell script that "fills in" the variables of the template and writes the generated file to disk.

Is this possible with a shell script. I would be very thankfull if you can name some commands/tools I can accomplish this or some good links.

This is very possible. A very simple way to implement this would be for the template file to actually be the script and use shell variables such as

#! /bin/bash
version="1.2.3"
path="/foo/bar/baz"
cat > /tmp/destfile <<-EOF
here is some config for version $version which should
also reference this path $path
EOF

You could even make this configurable on the command line by specifying version=$1 and path=$2, so you can run it like bash script /foo/bar/baz 1.2.3. The - before EOF causes whitespace before the lines be ignored, use plain <<EOF if you do not want that behavior.

Another way to do this would be to use the search and replace functionality of sed

#! /bin/bash
version="1.2.3"
path="/foo/bar/baz"
sed -e "s/VERSION/$version/g" -e "s/PATH/$path/" /path/to/templatefile > /tmp/destfile

which would replace each instance of the strings VERSION and PATH. If there are other reasons those strings would be in the template file you might make your search and replace be VERSION or %VERSION% or something less likely to be triggered accidentally.

The easiest way to do this simply in Linux CLI is to use envsubst and Environment Variables.

Example template file apache.tmpl:

<VirtualHost *:${PORT}>
    ServerName ${SERVER_NAME}
    ServerAlias ${SERVER_ALIAS}
    DocumentRoot "${DOCUMENT_ROOT}"
</VirtualHost>

Run envsubst and output result to new file my_apache_site.conf:

export PORT="443"
export SERVER_NAME="example.com"
export SERVER_ALIAS="www.example.com"
export DOCUMENT_ROOT="/var/www/html/"
envsubst < apache.tmpl > my_apache_site.conf

Output:

<VirtualHost *:443>
    ServerName example.com
    ServerAlias www.example.com
    DocumentRoot "/var/www/html/"
</VirtualHost>

No tools necessary other than /bin/sh. Given a template file of the form

Version: ${version}
Path: ${path}

or even with mixed shell code included

Version: ${version}
Path: ${path}
Cost: ${cost}\$
$(i=1; for w in one two three four; do echo Param${i}: ${w}; i=$(expr $i + 1); done)

and a shell parsable configuration file like

version="1.2.3-r42"
path="/some/place/under/the/rainbow/where/files/dance/in/happiness"
cost="42"

it is a simple matter to expand this to

Version: 1.2.3-r42
Path: /some/place/under/the/rainbow/where/files/dance/in/happiness
Cost: 42$
Param1: one
Param2: two
Param3: three
Param4: four

Indeed, given the path to the configuration file in shell variable config_file and the path to the template file in template_file, all you need to do is:

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

This is perhaps prettier than having complete shell script as the template file (@mtinberg's solution).

The complete naive template expander program:

#!/bin/sh

PROG=$(basename $0)

usage()
{
    echo "${PROG} <template-file> [ <config-file> ]"
}

expand()
{
    local template="$(cat $1)"
    eval "echo \"${template}\""
}

case $# in
    1) expand "$1";;
    2) . "$2"; expand "$1";;
    *) usage; exit 0;;
esac

This will output the expansion to standard output; just redirect standard output to a file or modify the above in obvious fashion to produce the desired output file.

Caveats: Template file expansion would not work if the file contained unescaped double quotes ("). For security reasons, we should probably include some obvious sanity checks or, even better, perform shell escaping transformation if the template file is generated by external entity.

You probably ought to look into a configuration management system like Puppet or Chef. These can easily do what you describe above and much more.

If you want lightweight and real templates rather than shell code that generates new files, the usual choices are sed& awk or perl. Here is one link: http://savvyadmin.com/generate-text-from-templates-scripts-and-csv-data/

Me, I'd use a real language like perl, tcl, python, ruby or something else in that class. Something built for scripting. They all have good, simple templating tools and tons of examples in google.

Templating with Linux in a Shell Script?

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