goenvsubst (Go Environment substitution; gopherified version of envsubst) provides a simple way to write environment variable values to files on container startup.
Let's say you have nginx in a container and simply want to dynamically set a value from the container environment to its configuration,
goenvsubst is an easy way to do that.
It supports
- Replace ENV Variable values, with optional default values (simple
${ENVNAME}or${ENVNAME:-default}with default value) - Full go templating support
By default (with no arguments), goenvsubst reads input from stdin:
echo 'This is my Template with ${VAR}:' > tmpl
export VAR="my dynamic value"
cat tmpl | goenvsubst > config.conf
cat config.conf
This is my Template with my dynamic value:
You can set default values inline. If the ENV variable is not set, that value will be set:
echo 'My value is ${VAR:-default} set.' > tmpl
cat tmpl | goenvsubst > config.conf
cat config.conf
My value is default set.
You can also pass the template file path as the first argument:
echo 'This is my Template with ${VAR}:' > tmpl
export VAR="my dynamic value"
goenvsubst ./tmpl > config.conf
cat config.conf
This is my Template with my dynamic value:
Before replacing any ${..} occurrences, the whole content is parsed as a Go template. Every ENV variable
is a variable in the template.
echo 'My name is {{.NAME}} {{ if .AGE }}and i am {{.AGE}} years old{{ end }} :' > tmpl
export NAME="Jeff"
export AGE=13
goenvsubst ./tmpl > config.conf
cat config.conf
My name is Jeff and i am 13 years old :
Just add the latest [prebuilt binary for 64-bit Linux][releases] from the Github releases page to your container:
Dockerfile
FROM ....
ENV GOENVSUBST_VERSION=v0.1.0
ADD https://github.com/libgraviton/goenvsubst/releases/download/${GOENVSUBST_VERSION}/goenvsubst-amd64 /sbin/goenvsubst
RUN chmod +x /sbin/goenvsubst
I did this for my own need. After doing it, I discovered a8m/envsubst which seems to provide a more mature set of checking features and more advanced syntax.
I contemplated first to delete this again. But then I realized that this small utility is a better match for many simple use cases and as it also supports Go templating (which a8m/envsubst does not), I decided to keep this.