envsubst - Substitute Variables for Environment Variables

Hi 👋,

Introduction

In this short article I want to showcase a nice and useful Linux command.

The comand 🥁 envsubst 🥁. In short it Substitutes the values of environment variables.

It's great for populating configuration files with values from environment variables, a common operation for developers containerizing their applications.

Example Usecase

Let's say we are containerizing an application and we have the following file configuration.yaml, and we want to modify the values of the environment field and the log_level field without adding the additional complexity of mounting the configuration.yaml file into the container/pod.

configuration:
  server: "the-app"
  environment: "production"
  log_level": "info"

To change the values of the environment and loglevel fields we create a configuration-template.yaml like so:

configuration:
    server: "the-app"
    environment: "$ENV_ENVIRONMENT"
    log_level": "$ENV_LOGLEVEL"

Then we run eventsubt to substitute the configuration values:

export ENV_ENVIRONMENT=stagging
export ENV_LOGLEVEL=trace

envsubst < configuration-template.yaml > configuration.yaml

The resulting configuration.yaml file will contain:

configuration:
    server: "the-app"
    environment: "stagging"
    log_level": "trace"%  

Conclusion

The envsubst command enables us to write configuration files with placeholders that will be replaced with actual values from the environment variables. Before being aware if this command I always thought that I have to somehow parse the files, substitute variables dynamically, ensure that the substitution didn't change the file format or break something. Now I just envsubst .

Thanks for reading and happy dev-ing! 🖥️🧑‍💻

envsubst help

Usage: envsubst [OPTION] [SHELL-FORMAT]

Substitutes the values of environment variables.

Operation mode:
-v, --variables output the variables occurring in SHELL-FORMAT

Informative output:
-h, --help display this help and exit
-V, --version output version information and exit

In normal operation mode, standard input is copied to standard output,
with references to environment variables of the form $VARIABLE or ${VARIABLE}
being replaced with the corresponding values. If a SHELL-FORMAT is given,
only those environment variables that are referenced in SHELL-FORMAT are
substituted; otherwise all environment variables references occurring in
standard input are substituted.

When --variables is used, standard input is ignored, and the output consists
of the environment variables that are referenced in SHELL-FORMAT, one per line.

Report bugs in the bug tracker at https://savannah.gnu.org/projects/gettext
or by email to bug-gettext@gnu.org.