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// 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))
// ${} is all ASCII, so bytes are fine for this operation.
i := 0
for j := 0; j < len(s); j++ {
if s[j] == '$' && j+1 < len(s) {
buf = append(buf, []byte(s[i:j])...)
name, w := getShellName(s[j+1:])
buf = append(buf, []byte(mapping(name))...)
j += w
i = j + 1
}
}
return string(buf) + s[i:]
}
// 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
}
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