www.nico.schottelius.org/blog/stdin-here-documents-templating-in-cdist.mdwn

[[!meta title="How to use stdin and here documents for templating in cdist"]]

## Introduction

In the shell you can see the use of
[here documents](http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_07_04) from time to time. They are very practical if you want to
feed some data with line breaks (also referred to as
"document") into another programm at the current position
("here") in the shell.

[[cdist|software/cdist]] allows you to make use of stdin and thus
also of here documents in your types. This article gives you some 
examples on how you can use them.

## Here documents in short

For those who have found this page, but are not familiar with here documents,
here is a short example of how you can use them:

    cat << eof
    Hello world,
    this is a here document.
    eof

The interesting part of here documents is that you can
use parameter expansion, command substitution, and arithmetic expansion
(as described in the **here documents** article linked above - opengroup
offers a great reference for shell coders/users):

    name="Nico"
    cat << eof
    Hello $name,

    1+1 = $((1+1))

    ls ~ = $(ls ~)

    eof

Just try it - copy and paste the above code into your shell and it will
display the result of 1+1 and the contents of your home directory.

## Here documents and stdin in cdist

Whenever you execute a type in a manifest in cdist like this:

    __file /tmp/testfile

cdist also reads stdin that is supplied to the type. 
Not every type that is shipped with cdist makes use
of stdin, but [__file](/software/cdist/man/latest/man7/cdist-type__file.html)
does (always check the manpage of the cdist types - if a type makes
use of stdin, it is documented in there).

Indeed, if **\_\_file** sees that you use "-" as the value for the
**source** parameter, it will use stdin for the content of the file
that it maintains:

    echo "Hello file" | __file /tmp/testfile --source -

Instead of using echo, we could also use the previously mentioned here document:

    __file /tmp/testfile --source - << eof
    Hello world,
    this is a here document.
    eof

Beware, you could use cat like this

    cat << eof | __file /tmp/testfile --source -
    Hello world,
    this is a here document.
    eof

but it is a
[useless use of cat (UUOC)](https://en.wikipedia.org/wiki/Cat_(Unix)#Useless_use_of_cat).

## Templating using here documents in cdist

Here documents are very powerful and they are very useful for templating. 
Indeed, the [__ungleich_nginx_site type](https://github.com/ungleich/cdist-examples/tree/master/type/__ungleich_nginx_site) uses a template like this in its manifest:

    template_in=$__type/files/nginx-template
    template_out=$__object/files/nginx-template

    export www_dir="$base_dir/www"
    export log_dir="$base_dir/logs"

    ... (including more exports)

    mkdir "$__object/files"
    sh -e "$template_in" > "$template_out"


The following code shows the template (**$__type/files/nginx-template**):

    cat << eof

    #
    # Do not change this file. Changes will be overwritten by cdist.
    #

    server {
       # Only bind on the reserved IP address
       listen $listen;

    eof

    servername="$name"

    for a in $alias; do
        servername="$servername $a"
    done
    servername="$servername"

    cat <<eof

        server_name $servername;

        location / {
            root $www_dir;
    eof

    if [ -f "$__object/parameter/locationopt" ]; then
        echo "    # User given location parameters"
        while read line; do
            echo "        $line"
        done < "$__object/parameter/locationopt"
    fi

    cat <<eof
        }

        access_log $log_dir/access.log;

    }

    eof


## Had fun?

The shell is indeed very powerful, you just need to know how to use it. 
This is why cdist was even originally written in shell script and is 
still configured in shell script (and will continue to be so).

If you are shell junkie, you may find more addictive drugs
[[in this blog|blog]].

[[!tag cdist config shell unix]]