envsubst

Environment variables substitution for Go. see docs below

Installation:

From binaries

Latest stable envsubst prebuilt binaries for 64-bit Linux, or Mac OS X are available via Github releases.

Linux and MacOS

curl -L https://github.com/a8m/envsubst/releases/download/v1.2.0/envsubst-`uname -s`-`uname -m` -o envsubst
chmod +x envsubst
sudo mv envsubst /usr/local/bin

Windows

Download the latest prebuilt binary from releases page, or if you have curl installed:

curl -L https://github.com/a8m/envsubst/releases/download/v1.2.0/envsubst.exe

With go

You can install via go get (provided you have installed go):

go get github.com/a8m/envsubst/cmd/envsubst

Using via cli

envsubst < input.tmpl > output.text
echo 'welcome $HOME ${USER:=a8m}' | envsubst
envsubst -help

Imposing restrictions

There are three command line flags with which you can cause the substitution to stop with an error code, should the restriction associated with the flag not be met. This can be handy if you want to avoid creating e.g. configuration files with unset or empty parameters. Setting a -fail-fast flag in conjunction with either no-unset or no-empty or both will result in a faster feedback loop, this can be especially useful when running through a large file or byte array input, otherwise a list of errors is returned.

The flags and their restrictions are:

Option	Meaning	Type	Default
-i	input file	string | stdin	stdin
-o	output file	string | stdout	stdout
-no-digit	do not replace variables starting with a digit, e.g. $1 and ${1}	flag	false
-no-unset	fail if a variable is not set	flag	false
-no-empty	fail if a variable is set but empty	flag	false
-fail-fast	fails at first occurrence of an error, if -no-empty or -no-unset flags were not specified this is ignored	flag	false

These flags can be combined to form tighter restrictions.

Using envsubst programmatically ?

You can take a look on _example/main or see the example below.

package main

import (
	"fmt"
	"github.com/a8m/envsubst"
)

func main() {
    input := "welcome $HOME"
    str, err := envsubst.String(input)
    // ...
    buf, err := envsubst.Bytes([]byte(input))
    // ...
    buf, err := envsubst.ReadFile("filename")
}

Docs

api docs here:

Expression	Meaning
${var}	Value of var (same as $var)
${var-$DEFAULT}	If var not set, evaluate expression as $DEFAULT
${var:-$DEFAULT}	If var not set or is empty, evaluate expression as $DEFAULT
${var=$DEFAULT}	If var not set, evaluate expression as $DEFAULT
${var:=$DEFAULT}	If var not set or is empty, evaluate expression as $DEFAULT
${var+$OTHER}	If var set, evaluate expression as $OTHER, otherwise as empty string
${var:+$OTHER}	If var set, evaluate expression as $OTHER, otherwise as empty string
$$var	Escape expressions. Result will be $var.

_{Most of the rows in this table were taken from here}

See also

• os.ExpandEnv(s string) string - only supports $var and ${var} notations

Creating Releases

This project uses automated workflows to create releases with prebuilt binaries for multiple platforms.

Release Workflows

create-release.yml: Creates git tags and GitHub releases

• Trigger: Manual via GitHub Actions UI
• Inputs: Tag name, optional release title and description
• Features:
  • Creates git tag (skips if already exists)
  • Creates GitHub release
  • Handles existing tags gracefully (rerunnable)

binaries.yml: Builds and uploads binaries

• Triggers:
  • Automatically on release creation
  • Manual dispatch with optional tag name
• Platforms: Linux (amd64, arm64), macOS (amd64, arm64), Windows (amd64)
• Features: Builds from specific tag or latest release

Release Procedure

1. Go to the Actions tab in the GitHub repository
2. Select the "Create Release" workflow
3. Click "Run workflow"
4. Enter the tag name following semantic versioning (e.g., v1.4.5)
5. Optionally provide a release title and description
6. Click "Run workflow"
7. Wait for the workflow to complete successfully
8. Go back to the Actions tab and select the "Release Binaries" workflow
9. Click "Run workflow" to build and upload binaries for the new release
10. Enter the same tag name in step (4) (or select Use workflow from released tag)
11. Click "Run workflow"

What Happens During Release

1. Tag Creation: Creates a git tag with the specified version
2. Release Creation: Creates a GitHub release with optional title/description
3. Binary Building: Automatically triggers binary builds for all platforms:
   • envsubst-Linux-x86_64 (Linux AMD64)
   • envsubst-Linux-arm64 (Linux ARM64)
   • envsubst-Darwin-x86_64 (macOS Intel)
   • envsubst-Darwin-arm64 (macOS Apple Silicon)
   • envsubst (Windows AMD64)
4. Asset Upload: Binaries are automatically attached to the release

Rerunning Releases

• Same Tag: Both workflows can be rerun with the same tag name
• Tag Exists: The create-release workflow will skip tag creation if it already exists
• Release Exists: The workflow continues even if the release already exists
• Binary Rebuild: Use the binaries workflow to rebuild assets for existing releases

Supported Platforms

Platform	Architecture	Binary Name
Linux	AMD64	envsubst-Linux-x86_64
Linux	ARM64	envsubst-Linux-arm64
macOS	Intel	envsubst-Darwin-x86_64
macOS	Apple Silicon	envsubst-Darwin-arm64
Windows	AMD64	envsubst

License

MIT