goenvtemplator

README

goenvtemplator

Tool to template configuration files by environment variables and optionally replace itself with the target binary.

goenvtemplator is a simple app, that can template your config files by environment variables and optionally replace itself (by exec syscall) with the application binary. So at the end your application runs directly under the process that run this tool like docker as if it was originally the entrypoint itself.

This tool is ideal for use without polluting you environment with dependencies. It is fully statically linked so it has no dependencies whatsoever. If you use Dockerfile you don't even need wget or curl since it can be installed only by dockerfile's ADD instruction.

Installation

wget

wget https://github.com/seznam/goenvtemplator/releases/download/v1.0.0/goenvtemplator-amd64 -O /usr/local/bin/goenvtemplator
chmod +x /usr/local/bin/goenvtemplator

Dockerfile

ADD https://github.com/seznam/goenvtemplator/releases/download/v1.0.0/goenvtemplator-amd64 /usr/local/bin/goenvtemplator
RUN chmod +x /usr/local/bin/goenvtemplator

Usage

goenvtemplator -help

Usage of goenvtemplator:
  -debug-templates
    	Print processed templates to stdout.
  -env-file value
        Additional file with environment variables. Can be passed multiple times. (default [])
  -exec
    	Activates exec by command. First non-flag arguments is the command, the rest are it's arguments.
  -template value
    	Template (/template:/dest). Can be passed multiple times. (default [])
  -v int
    	Verbosity level.
  -version
    	Prints version.

Example

goenvtemplator -template /path/to/server.conf.tmpl:/path/to/server.conf  -template /path/to/server2.conf.tmpl:/path/to/server2.conf

Dockerfile

ENTRYPOINT ["/usr/local/bin/goenvtemplator", "-template", "/path/to/server.conf.tmpl:/path/to/server.conf", "-exec"]
CMD ["/usr/bin/server-binary", "server-argument1", "server-argument2", "..."]

Env files

It is possible to add additional environment variables in multiple env-files. Existing variables are not overwritten. Environment variables in files can be formated using shell syntax or yaml syntax.

Let us consider an environment file myenvfile bellow.

# cat myenvfile
A=a
B=b
B=bb
#B=bbb
export C=c
# yaml syntax
D: d

The behaviour of env-file argument and env variables evaluation is as follows:

goenvtemplator -env-file myenvfile -exec sh -c 'echo $A'
> a
export A=foo
goenvtemplator -env-file myenvfile -exec sh -c 'echo $A'
> foo
goenvtemplator -env-file myenvfile -exec sh -c 'echo $B'
> bb
goenvtemplator -env-file myenvfile -exec sh -c 'echo $C'
> c
goenvtemplator -env-file myenvfile -exec sh -c 'echo $D'
> d

Using Templates

Templates use Golang text/template.

Built-in functions

There are a few built in functions as well:

• env "ENV_NAME" - Accesses environment variables. If it does not exist return empty string. {{ env "TIMEOUT_MS }}
• require (env "ENV_NAME") - Renders an error if environments variable does not exists. If it is equal to empty string, returns empty string. {{ require (env "TIMEOUT_MS) }}
• default $default_1 $default_2 $default_3 - Returns a first argument that exists. If none is valid it generates error {{ default (env "SPECIFIC_TIMEOUT_MS") (env "GENERAL_TIMEOUT_MS") "1000" }}