150 lines
4 KiB
Markdown
150 lines
4 KiB
Markdown
[[!meta title="How to use stdin and here documents for templating in cdist"]]
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## Introduction
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In the shell you can see the use of
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[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
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feed some data with line breaks (also referred to as
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"document") into another programm at the current position
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("here") in the shell.
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[[cdist|software/cdist]] allows you to make use of stdin and thus
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also of here documents in your types. This article gives you some
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examples on how you can use them.
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## Here documents in short
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For those who have found this page, but are not familiar with here documents,
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here is a short example of how you can use them:
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cat << eof
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Hello world,
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this is a here document.
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eof
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The interesting part of here documents is that you can
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use parameter expansion, command substitution, and arithmetic expansion
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(as described in the **here documents** article linked above - opengroup
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offers a great reference for shell coders/users):
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name="Nico"
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cat << eof
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Hello $name,
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1+1 = $((1+1))
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ls ~ = $(ls ~)
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eof
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Just try it - copy and paste the above code into your shell and it will
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display the result of 1+1 and the contents of your home directory.
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## Here documents and stdin in cdist
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Whenever you execute a type in a manifest in cdist like this:
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__file /tmp/testfile
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cdist also reads stdin that is supplied to the type.
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Not every type that is shipped with cdist makes use
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of stdin, but [__file](/software/cdist/man/latest/man7/cdist-type__file.html)
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does (always check the manpage of the cdist types - if a type makes
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use of stdin, it is documented in there).
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Indeed, if **\_\_file** sees that you use "-" as the value for the
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**source** parameter, it will use stdin for the content of the file
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that it maintains:
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echo "Hello file" | __file /tmp/testfile --source -
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Instead of using echo, we could also use the previously mentioned here document:
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__file /tmp/testfile --source - << eof
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Hello world,
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this is a here document.
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eof
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Beware, you could use cat like this
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cat << eof | __file /tmp/testfile --source -
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Hello world,
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this is a here document.
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eof
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but it is a
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[useless use of cat (UUOC)](https://en.wikipedia.org/wiki/Cat_(Unix)#Useless_use_of_cat).
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## Templating using here documents in cdist
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Here documents are very powerful and they are very useful for templating.
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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:
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template_in=$__type/files/nginx-template
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template_out=$__object/files/nginx-template
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export www_dir="$base_dir/www"
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export log_dir="$base_dir/logs"
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... (including more exports)
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mkdir "$__object/files"
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sh -e "$template_in" > "$template_out"
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The following code shows the template (**$__type/files/nginx-template**):
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cat << eof
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#
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# Do not change this file. Changes will be overwritten by cdist.
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#
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server {
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# Only bind on the reserved IP address
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listen $listen;
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eof
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servername="$name"
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for a in $alias; do
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servername="$servername $a"
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done
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servername="$servername"
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cat <<eof
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server_name $servername;
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location / {
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root $www_dir;
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eof
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if [ -f "$__object/parameter/locationopt" ]; then
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echo " # User given location parameters"
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while read line; do
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echo " $line"
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done < "$__object/parameter/locationopt"
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fi
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cat <<eof
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}
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access_log $log_dir/access.log;
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}
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eof
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## Had fun?
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The shell is indeed very powerful, you just need to know how to use it.
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This is why cdist was even originally written in shell script and is
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still configured in shell script (and will continue to be so).
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If you are shell junkie, you may find more addictive drugs
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[[in this blog|blog]].
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[[!tag cdist config shell unix]]
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