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os: add Expand function to evaluate environment variables.
Fixes #1258. R=gri, msolo, bradfitzpatrick, r2 CC=golang-dev https://golang.org/cl/2988041
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@ -8,6 +8,7 @@ TARG=os
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GOFILES=\
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dir_$(GOOS).go\
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error.go\
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env.go\
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exec.go\
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file.go\
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getwd.go\
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73
src/pkg/os/env.go
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73
src/pkg/os/env.go
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@ -0,0 +1,73 @@
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// Copyright 2010 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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// General environment variables.
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package os
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// Expand replaces ${var} or $var in the string based on the mapping function.
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// Invocations of undefined variables are replaced with the empty string.
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func Expand(s string, mapping func(string) string) string {
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buf := make([]byte, 0, 2*len(s))
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// ${} is all ASCII, so bytes are fine for this operation.
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for i := 0; i < len(s); {
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if s[i] != '$' || i == len(s)-1 {
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buf = append(buf, s[i])
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i++
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continue
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}
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name, w := getShellName(s[i+1:])
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buf = append(buf, []byte(mapping(name))...)
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i += 1 + w
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}
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return string(buf)
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}
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// ShellExpand replaces ${var} or $var in the string according to the values
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// of the operating system's environment variables. References to undefined
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// variables are replaced by the empty string.
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func ShellExpand(s string) string {
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return Expand(s, Getenv)
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}
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// isSpellSpecialVar reports whether the character identifies a special
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// shell variable such as $*.
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func isShellSpecialVar(c uint8) bool {
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switch c {
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case '*', '#', '$', '@', '!', '?', '0', '1', '2', '3', '4', '5', '6', '7', '8', '9':
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return true
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}
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return false
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}
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// isAlphaNum reports whether the byte is an ASCII letter, number, or underscore
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func isAlphaNum(c uint8) bool {
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return c == '_' || '0' <= c && c <= '9' || 'a' <= c && c <= 'z' || 'A' <= c && c <= 'Z'
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}
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// getName returns the name that begins the string and the number of bytes
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// consumed to extract it. If the name is enclosed in {}, it's part of a ${}
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// expansion and two more bytes are needed than the length of the name.
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func getShellName(s string) (string, int) {
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switch {
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case s[0] == '{':
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if len(s) > 2 && isShellSpecialVar(s[1]) && s[2] == '}' {
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return s[1:2], 3
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}
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// Scan to closing brace
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for i := 1; i < len(s); i++ {
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if s[i] == '}' {
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return s[1:i], i + 1
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}
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}
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return "", 1 // Bad syntax; just eat the brace.
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case isShellSpecialVar(s[0]):
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return s[0:1], 1
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}
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// Scan alphanumerics.
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var i int
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for i = 0; i < len(s) && isAlphaNum(s[i]); i++ {
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}
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return s[:i], i
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}
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59
src/pkg/os/env_test.go
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59
src/pkg/os/env_test.go
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@ -0,0 +1,59 @@
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// Copyright 2010 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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package os_test
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import (
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. "os"
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"testing"
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)
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// testGetenv gives us a controlled set of variables for testing Expand.
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func testGetenv(s string) string {
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switch s {
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case "*":
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return "all the args"
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case "#":
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return "NARGS"
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case "$":
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return "PID"
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case "1":
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return "ARGUMENT1"
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case "HOME":
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return "/usr/gopher"
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case "H":
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return "(Value of H)"
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case "home_1":
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return "/usr/foo"
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case "_":
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return "underscore"
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}
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return ""
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}
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var expandTests = []struct {
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in, out string
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}{
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{"", ""},
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{"$*", "all the args"},
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{"$$", "PID"},
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{"${*}", "all the args"},
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{"$1", "ARGUMENT1"},
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{"${1}", "ARGUMENT1"},
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{"now is the time", "now is the time"},
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{"$HOME", "/usr/gopher"},
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{"$home_1", "/usr/foo"},
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{"${HOME}", "/usr/gopher"},
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{"${H}OME", "(Value of H)OME"},
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{"A$$$#$1$H$home_1*B", "APIDNARGSARGUMENT1(Value of H)/usr/foo*B"},
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}
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func TestExpand(t *testing.T) {
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for _, test := range expandTests {
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result := Expand(test.in, testGetenv)
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if result != test.out {
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t.Errorf("Expand(%q)=%q; expected %q", test.in, result, test.out)
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}
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}
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}
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