Fichier de configuration simple paramétré dans Docker

Parameterized config files are a must to create customizable and flexible docker images. There are many tools available for doing the same. To name a few, consul-template, confd

But in this article I want to discuss about few of the basic approaches. So why basic ones? Well, when you can do the job with a knife, why use a gun!

Approach 1 - Using Bash Substituion

This uses shell parameter and file redirection. Consider we have a templated file
conf.txt

My name is $NAME or ${NAME} ?

Now we write a bash function to parse and output the parsed text to a destination file

$ parse_template_file(){
local FILE=$1
local OUT=$2
eval "cat <<EOF
$(cat $FILE)
EOF" > $OUT
}

$ NAME=TARUN parse_template_file conf.txt /dev/stdout
My name is TARUN or TARUN ?
$ NAME=TARUN parse_template_file conf.txt conf2.txt
$ cat conf2.txt
My name is TARUN or TARUN ?

This works in all environments as just uses the shell’s functionality. The two type of parameter you can use is the form of $NAME or ${NAME}

Note: If you need to have a literal $NAME in your output then use \$NAME

Approach 2 - Using envsubst

envsubst command can be used to parse environment variables passed in standard input and output the parsed values on standard output

So our new commands from previous example would now look like

$ NAME=TARUN envsubst < conf.txt
My name is TARUN or TARUN ?
$ NAME=TARUN envsubst < conf.txt > conf3.txt
$ cat conf3.txt
My name is TARUN or TARUN ?

envsubst is not a shell command. It is provided by a gettext package

$ dpkg -S /usr/bin/envsubst
gettext-base: /usr/bin/envsubst

So this may not be available on all the docker images, you may need to install it