bb

Go Busybox

bb.go in this package implements a Go source-to-source transformation on pure Go code (no cgo).

This AST transformation does the following:

• Takes a Go command's source files and rewrites them into Go package files without global side effects.
• Writes a main.go file with a main() that calls into the appropriate Go command package based on argv[0].

This allows you to take two Go commands, such as Go implementations of sl and cowsay and compile them into one binary.

Which command is invoked is determined by argv[0] or argv[1] if argv[0] is not recognized. Let's say bb is the compiled binary; the following are equivalent invocations of sl and cowsay:

# Make a symlink sl -> bb
ln -s bb sl
./sl -l

# Make a symlink cowsay -> bb
ln -s bb cowsay
./cowsay Haha

./bb sl -l
./bb cowsay Haha

AST Transformation

Principally, the AST transformation moves all global side-effects into callable package functions. E.g. main becomes Main, each init becomes InitN, and global variable assignments are moved into their own InitN.

Then, these Main and Init functions can be registered with a global map of commands by name and used when called upon.

Let's say a command github.com/org/repo/cmds/sl contains the following main.go:

package main

import (
  "flag"
  "log"
)

var name = flag.String("name", "", "Gimme name")

func init() {
  log.Printf("init %s", *name)
}

func main() {
  log.Printf("train")
}

This would be rewritten to be:

package sl // based on the directory name or bazel-rule go_binary name

import (
  "flag"
  "log"

  // This package holds the global map of commands.
  "github.com/u-root/u-root/pkg/bb"
)

// Type has to be inferred through type checking.
var name *string

func Init0() {
  log.Printf("init %s", *name)
}

func Init1() {
  name = flag.String("name", "", "Gimme name")
}

func Init() {
  // Order is determined by go/types.Info.InitOrder.
  Init1()
  Init0()
}

func Main() {
  log.Printf("train")
}

func init() {
  // Register `sl` as a command.
  bb.Register("sl", Init, Main)
}

Shortcomings

• If there is already a function Main or InitN for some N, there may be a compilation error.
• Any packages imported by commands may still have global side-effects affecting other commands. Done properly, we would have to rewrite all non-standard-library packages as well as commands. This has not been necessary to implement so far. It would likely be necessary if two different imported packages register the same flag unconditionally globally.

Generated main

The main file can be generated based on any template Go files, but the default looks something like the following:

import (
  "os"

  "github.com/u-root/u-root/pkg/bb"

  // Side-effect import registers command with bb.
  _ "github.com/org/repo/cmds/generated/sl"
)

func main() {
  bb.Run(os.Argv[0])
}

The default template will use argv[1] if argv[0] is not in the map.