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Source file src/os/env.go

     1	// Copyright 2010 The Go Authors. All rights reserved.
     2	// Use of this source code is governed by a BSD-style
     3	// license that can be found in the LICENSE file.
     4	
     5	// General environment variables.
     6	
     7	package os
     8	
     9	import "syscall"
    10	
    11	// Expand replaces ${var} or $var in the string based on the mapping function.
    12	// For example, os.ExpandEnv(s) is equivalent to os.Expand(s, os.Getenv).
    13	func Expand(s string, mapping func(string) string) string {
    14		buf := make([]byte, 0, 2*len(s))
    15		// ${} is all ASCII, so bytes are fine for this operation.
    16		i := 0
    17		for j := 0; j < len(s); j++ {
    18			if s[j] == '$' && j+1 < len(s) {
    19				buf = append(buf, s[i:j]...)
    20				name, w := getShellName(s[j+1:])
    21				buf = append(buf, mapping(name)...)
    22				j += w
    23				i = j + 1
    24			}
    25		}
    26		return string(buf) + s[i:]
    27	}
    28	
    29	// ExpandEnv replaces ${var} or $var in the string according to the values
    30	// of the current environment variables. References to undefined
    31	// variables are replaced by the empty string.
    32	func ExpandEnv(s string) string {
    33		return Expand(s, Getenv)
    34	}
    35	
    36	// isShellSpecialVar reports whether the character identifies a special
    37	// shell variable such as $*.
    38	func isShellSpecialVar(c uint8) bool {
    39		switch c {
    40		case '*', '#', '$', '@', '!', '?', '-', '0', '1', '2', '3', '4', '5', '6', '7', '8', '9':
    41			return true
    42		}
    43		return false
    44	}
    45	
    46	// isAlphaNum reports whether the byte is an ASCII letter, number, or underscore
    47	func isAlphaNum(c uint8) bool {
    48		return c == '_' || '0' <= c && c <= '9' || 'a' <= c && c <= 'z' || 'A' <= c && c <= 'Z'
    49	}
    50	
    51	// getShellName returns the name that begins the string and the number of bytes
    52	// consumed to extract it. If the name is enclosed in {}, it's part of a ${}
    53	// expansion and two more bytes are needed than the length of the name.
    54	func getShellName(s string) (string, int) {
    55		switch {
    56		case s[0] == '{':
    57			if len(s) > 2 && isShellSpecialVar(s[1]) && s[2] == '}' {
    58				return s[1:2], 3
    59			}
    60			// Scan to closing brace
    61			for i := 1; i < len(s); i++ {
    62				if s[i] == '}' {
    63					return s[1:i], i + 1
    64				}
    65			}
    66			return "", 1 // Bad syntax; just eat the brace.
    67		case isShellSpecialVar(s[0]):
    68			return s[0:1], 1
    69		}
    70		// Scan alphanumerics.
    71		var i int
    72		for i = 0; i < len(s) && isAlphaNum(s[i]); i++ {
    73		}
    74		return s[:i], i
    75	}
    76	
    77	// Getenv retrieves the value of the environment variable named by the key.
    78	// It returns the value, which will be empty if the variable is not present.
    79	// To distinguish between an empty value and an unset value, use LookupEnv.
    80	func Getenv(key string) string {
    81		v, _ := syscall.Getenv(key)
    82		return v
    83	}
    84	
    85	// LookupEnv retrieves the value of the environment variable named
    86	// by the key. If the variable is present in the environment the
    87	// value (which may be empty) is returned and the boolean is true.
    88	// Otherwise the returned value will be empty and the boolean will
    89	// be false.
    90	func LookupEnv(key string) (string, bool) {
    91		return syscall.Getenv(key)
    92	}
    93	
    94	// Setenv sets the value of the environment variable named by the key.
    95	// It returns an error, if any.
    96	func Setenv(key, value string) error {
    97		err := syscall.Setenv(key, value)
    98		if err != nil {
    99			return NewSyscallError("setenv", err)
   100		}
   101		return nil
   102	}
   103	
   104	// Unsetenv unsets a single environment variable.
   105	func Unsetenv(key string) error {
   106		return syscall.Unsetenv(key)
   107	}
   108	
   109	// Clearenv deletes all environment variables.
   110	func Clearenv() {
   111		syscall.Clearenv()
   112	}
   113	
   114	// Environ returns a copy of strings representing the environment,
   115	// in the form "key=value".
   116	func Environ() []string {
   117		return syscall.Environ()
   118	}
   119	

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