os: add Expand function to evaluate environment variables.

Fixes #1258.

R=gri, msolo, bradfitzpatrick, r2
CC=golang-dev
https://golang.org/cl/2988041

src/pkg/os/Makefile

@@ -8,6 +8,7 @@ TARG=os
 GOFILES=\
 	dir_$(GOOS).go\
 	error.go\
+	env.go\
 	exec.go\
 	file.go\
 	getwd.go\

src/pkg/os/env.go

+// Copyright 2010 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// General environment variables.
+
+package os
+
+// Expand replaces ${var} or $var in the string based on the mapping function.
+// Invocations of undefined variables are replaced with the empty string.
+func Expand(s string, mapping func(string) string) string {
+	buf := make([]byte, 0, 2*len(s))
+	// ${} is all ASCII, so bytes are fine for this operation.
+	for i := 0; i < len(s); {
+		if s[i] != '$' || i == len(s)-1 {
+			buf = append(buf, s[i])
+			i++
+			continue
+		}
+		name, w := getShellName(s[i+1:])
+		buf = append(buf, []byte(mapping(name))...)
+		i += 1 + w
+	}
+	return string(buf)
+}
+
+// ShellExpand replaces ${var} or $var in the string according to the values
+// of the operating system's environment variables.  References to undefined
+// variables are replaced by the empty string.
+func ShellExpand(s string) string {
+	return Expand(s, Getenv)
+}
+
+// isSpellSpecialVar reports whether the character identifies a special
+// shell variable such as $*.
+func isShellSpecialVar(c uint8) bool {
+	switch c {
+	case '*', '#', '$', '@', '!', '?', '0', '1', '2', '3', '4', '5', '6', '7', '8', '9':
+		return true
+	}
+	return false
+}
+
+// isAlphaNum reports whether the byte is an ASCII letter, number, or underscore
+func isAlphaNum(c uint8) bool {
+	return c == '_' || '0' <= c && c <= '9' || 'a' <= c && c <= 'z' || 'A' <= c && c <= 'Z'
+}
+
+// getName returns the name that begins the string and the number of bytes
+// consumed to extract it.  If the name is enclosed in {}, it's part of a ${}
+// expansion and two more bytes are needed than the length of the name.
+func getShellName(s string) (string, int) {
+	switch {
+	case s[0] == '{':
+		if len(s) > 2 && isShellSpecialVar(s[1]) && s[2] == '}' {
+			return s[1:2], 3
+		}
+		// Scan to closing brace
+		for i := 1; i < len(s); i++ {
+			if s[i] == '}' {
+				return s[1:i], i + 1
+			}
+		}
+		return "", 1 // Bad syntax; just eat the brace.
+	case isShellSpecialVar(s[0]):
+		return s[0:1], 1
+	}
+	// Scan alphanumerics.
+	var i int
+	for i = 0; i < len(s) && isAlphaNum(s[i]); i++ {
+	}
+	return s[:i], i
+}

src/pkg/os/env_test.go

+// Copyright 2010 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package os_test
+
+import (
+	. "os"
+	"testing"
+)
+
+// testGetenv gives us a controlled set of variables for testing Expand.
+func testGetenv(s string) string {
+	switch s {
+	case "*":
+		return "all the args"
+	case "#":
+		return "NARGS"
+	case "$":
+		return "PID"
+	case "1":
+		return "ARGUMENT1"
+	case "HOME":
+		return "/usr/gopher"
+	case "H":
+		return "(Value of H)"
+	case "home_1":
+		return "/usr/foo"
+	case "_":
+		return "underscore"
+	}
+	return ""
+}
+
+var expandTests = []struct {
+	in, out string
+}{
+	{"", ""},
+	{"$*", "all the args"},
+	{"$$", "PID"},
+	{"${*}", "all the args"},
+	{"$1", "ARGUMENT1"},
+	{"${1}", "ARGUMENT1"},
+	{"now is the time", "now is the time"},
+	{"$HOME", "/usr/gopher"},
+	{"$home_1", "/usr/foo"},
+	{"${HOME}", "/usr/gopher"},
+	{"${H}OME", "(Value of H)OME"},
+	{"A$$$#$1$H$home_1*B", "APIDNARGSARGUMENT1(Value of H)/usr/foo*B"},
+}
+
+func TestExpand(t *testing.T) {
+	for _, test := range expandTests {
+		result := Expand(test.in, testGetenv)
+		if result != test.out {
+			t.Errorf("Expand(%q)=%q; expected %q", test.in, result, test.out)
+		}
+	}
+}