[[!meta title="Using stdin and here documents in cdist for templating"]] ## 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 in 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 ~ = $(ls ~) eof ## Here documents and stdin in cdist Whenever you execute a type in a manifest in cdist like this: __file /tmp/testfile cdist also reads stdin that is supplied to the type. Not every type that is shipped with cdist makes use of stdin, but [__file](/software/cdist/man/latest/man7/cdist-type__file.html) does (always check the manpage of our types - if a type makes use of stdin, it is documented in there). Indeed, if **\_\_file** sees that you use "-" as the value for the **source** parameter, it will use stdin for the content of the file that it maintains: echo "Hello file" | __file /tmp/testfile --source - Instead of using echo, we could also use the previously mentioned here document: __file /tmp/testfile --source - << eof Hello world, this is a here document. eof Beware, you could use cat like this cat << eof | __file /tmp/testfile --source - Hello world, this is a here document. eof but it is a ([useless use of cat (UUOC)](https://en.wikipedia.org/wiki/Cat_(Unix)#Useless_use_of_cat). ## Templating using here documents in cdist Here documents are very powerful and they are very useful for templating. Indeed, the **__ungleich_nginx_site** type uses a template like this in its manifest: template_in=$__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 <