go-subst

go-subst is a CLI tool that embeds environment variables into text files, used for similar purposes as envsubst, but with enhanced safety.

Features

• Support Go Template Custom Functions: env and must_env.
  • {{ env `ENVVAR` }}: Fetches the environment variable ENVVAR. If it is not set, the output remains unchanged.
  • {{ must_env `ENVVAR` }}: If the environment variable is not set, go-subst will return an error.

Installation

Download binary from GitHub releases.

or

go install github.com/ToshihitoKon/go-subst/cmd/go-subst@latest

Usage

export SOME_ENV=env-value
cat template | go-subst > output-file

Examples

Template file example.service.tmpl

[Unit]
Description={{ env `SERVICE_DESCRIPTION` }}
After=network.service

[Service]
Type=simple
ExecStart={{ must_env `SERVICE_BIN_PATH` }}
ExecStop=/bin/kill -WINCH ${MAINPID}
Restart=always

[Install]
WantedBy=multi-user.target

Generate service file using go-subst.

 $ export SERVICE_DESCRIPTION="This is example service." SERVICE_BIN_PATH="/usr/local/bin/example"
 $ cat example.service.tmpl | go-subst
[Unit]
Description=This is example service.
After=network.service

[Service]
Type=simple
ExecStart=/usr/local/bin/example
ExecStop=/bin/kill -WINCH ${MAINPID}
Restart=always

[Install]
WantedBy=multi-user.target

Lisence

This project is licensed under the MIT License, see the LICENSE and CREDITS files for details.

Motivation

Inspired by and using kayac/go-config as a package.