envsub

envsub

envsub is a text preprocessing tool that performs environment variable substitution in files, with support for default values. Written in Rust and callable from Python, it provides a fast and reliable way to inject environment variables into text files during runtime.

Features

• Environment variable substitution in any text file.
• Support for default values when environment variables are missing.
• Fast performance with Rust under the hood, accessible from Python.

Installation

pip install envsub

Usage

envsub api

envsub comes with a sub method that text a io.TextIO and return a io.TextIO containing the replaced variable.

Basic usage:

from envsub import sub


with open("/path/to/file", "r") as downstream:
    with sub(downstream) as upstream:
        upstream.read()

When it is usefull.

envsub is made for replacing a set of variable inside a configuration file from envisonment variable, its feet well with confifuration file format.

Example with json:

import json
from envsub import sub

with open("/path/to/file.json", "r") as downstream:
    with sub(downstream) as upstream:
        data = json.load(upstream)

Substitution format

envsubst subsitute variable format that are curly-braced, like in bash.

{"hello": "${NAME}"}

⚠️ Warning:

The variable stay in the same line, no \n are permitted.**

This will not work: {"hello": "${ NAME }" }

Default value:

If the environment variable is not present, no substitution will be made, it means than in the previous example, ${NAME} will stay in the read value.

Alternatively, a default value can be set in the variable, using a - value.

{"hello": "${NAME-world}"}

In this case the final result will be {"hello": "world"}.

Alternatives

Similar tools exists, but did not find any good one in python.

The envsub lib has been created to replace non existing tool like a8m/envsubst a Go binary that update the GNU envsubst that do not support default values.