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

License

MIT