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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
This commit is contained in:
Rob Pike 2010-11-08 13:25:01 -08:00
parent fbd92dba96
commit 26cda27b0f
3 changed files with 133 additions and 0 deletions

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

73
src/pkg/os/env.go Normal file
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@ -0,0 +1,73 @@
// 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
}

59
src/pkg/os/env_test.go Normal file
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@ -0,0 +1,59 @@
// 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)
}
}
}