Interpolate a string with bash-like environment variables references

Question

I have an input string for my Golang CLI tool with some references to environment variables in bash syntax ($VAR and ${VAR}), e.g.:

$HOME/somedir/${SOME_VARIABLE}dir/anotherdir-${ANOTHER_VARIABLE}

What is the most efficient way to interpolate this string by replacing environemnt variables references with actual values? For previous example it can be:

/home/user/somedir/4dir/anotherdir-5

if HOME=/home/user, SOME_VARIABLE=4 and ANOTHER_VARIABLE=5.

Right now I'm using something like:

func interpolate(str string) string {
        for _, e := range os.Environ() {
                parts := strings.SplitN(e, "=", 2)
                name, value := parts[0], parts[1]
                str = strings.Replace(str, "$" + name, value, -1)
        }
        return str
}

but this code doesn't handle ${} variables.

Answer 1

Use os.ExpandEnv:

s := os.ExpandEnv(
    "$HOME/somedir/${SOME_VARIABLE}dir/anotherdir-${ANOTHER_VARIABLE}")

You can use os.Expand for the case where the values are not taken from environment variables:

m := map[string]string{
    "HOME":             "/home/user",
    "SOME_VARIABLE":    "4",
    "ANOTHER_VARIABLE": "5",
}
s := os.Expand(
    "$HOME/somedir/${SOME_VARIABLE}dir/anotherdir-${ANOTHER_VARIABLE}", 
    func(s string) string { return m[s] })

Run it in the playground.