rsperl / go-build-time-vars

Embed build-time variables using go:generate and go:embed

Build-Time Vars with go:generate

The go:generate comment

The generate comment must start at the beginning of the line with no space between // and go:generate:

//go:generate ...

This comment will be treated like an ordinary comment (ignored)

   //go:generate ...

Generating Files to Embed

When go generate . is run, go executes the command after the go:generate by treats the first word as the program and all other tokens after that as arguments to the program. It does not recognize > as shell redirection, since this is not being run by the shell. To get around this, use bash as the program to call, and put the actual command quoted in -c:

//go:generate bash -c "date --iso-8601=seconds > tmp/build_time"

Stripping Newlines

Because go:embed adds a new line to whatever it reads in, the following pattern is used:

// 1. Create the file in a file with the same name as the var that receives the value
// 2. embed in a lowercase variable
// 3. Set the titlecase variable to the trimmed result of the lowercase variable

//go:generate bash -c "date > tmp/varName"
//go:embed tmp/varName
var varName string

// VarName with upper case first letter is the real var, but
// the lowercase var actually receives the embed value
var VarName = strings.TrimRight(varName, "\n")