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moby--moby/builder/shell_parser.go

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package builder
// This will take a single word and an array of env variables and
// process all quotes (" and ') as well as $xxx and ${xxx} env variable
// tokens. Tries to mimic bash shell process.
// It doesn't support all flavors of ${xx:...} formats but new ones can
// be added by adding code to the "special ${} format processing" section
import (
"fmt"
"strings"
"unicode"
)
type shellWord struct {
word string
envs []string
pos int
}
// ProcessWord will use the 'env' list of environment variables,
// and replace any env var references in 'word'.
func ProcessWord(word string, env []string) (string, error) {
sw := &shellWord{
word: word,
envs: env,
pos: 0,
}
return sw.process()
}
func (sw *shellWord) process() (string, error) {
return sw.processStopOn('\000')
}
// Process the word, starting at 'pos', and stop when we get to the
// end of the word or the 'stopChar' character
func (sw *shellWord) processStopOn(stopChar rune) (string, error) {
var result string
var charFuncMapping = map[rune]func() (string, error){
'\'': sw.processSingleQuote,
'"': sw.processDoubleQuote,
'$': sw.processDollar,
}
for sw.pos < len(sw.word) {
ch := sw.peek()
if stopChar != '\000' && ch == stopChar {
sw.next()
break
}
if fn, ok := charFuncMapping[ch]; ok {
// Call special processing func for certain chars
tmp, err := fn()
if err != nil {
return "", err
}
result += tmp
} else {
// Not special, just add it to the result
ch = sw.next()
if ch == '\\' {
// '\' escapes, except end of line
ch = sw.next()
if ch == '\000' {
continue
}
}
result += string(ch)
}
}
return result, nil
}
func (sw *shellWord) peek() rune {
if sw.pos == len(sw.word) {
return '\000'
}
return rune(sw.word[sw.pos])
}
func (sw *shellWord) next() rune {
if sw.pos == len(sw.word) {
return '\000'
}
ch := rune(sw.word[sw.pos])
sw.pos++
return ch
}
func (sw *shellWord) processSingleQuote() (string, error) {
// All chars between single quotes are taken as-is
// Note, you can't escape '
var result string
sw.next()
for {
ch := sw.next()
if ch == '\000' || ch == '\'' {
break
}
result += string(ch)
}
return result, nil
}
func (sw *shellWord) processDoubleQuote() (string, error) {
// All chars up to the next " are taken as-is, even ', except any $ chars
// But you can escape " with a \
var result string
sw.next()
for sw.pos < len(sw.word) {
ch := sw.peek()
if ch == '"' {
sw.next()
break
}
if ch == '$' {
tmp, err := sw.processDollar()
if err != nil {
return "", err
}
result += tmp
} else {
ch = sw.next()
if ch == '\\' {
chNext := sw.peek()
if chNext == '\000' {
// Ignore \ at end of word
continue
}
if chNext == '"' || chNext == '$' {
// \" and \$ can be escaped, all other \'s are left as-is
ch = sw.next()
}
}
result += string(ch)
}
}
return result, nil
}
func (sw *shellWord) processDollar() (string, error) {
sw.next()
ch := sw.peek()
if ch == '{' {
sw.next()
name := sw.processName()
ch = sw.peek()
if ch == '}' {
// Normal ${xx} case
sw.next()
return sw.getEnv(name), nil
}
if ch == ':' {
// Special ${xx:...} format processing
// Yes it allows for recursive $'s in the ... spot
sw.next() // skip over :
modifier := sw.next()
word, err := sw.processStopOn('}')
if err != nil {
return "", err
}
// Grab the current value of the variable in question so we
// can use to to determine what to do based on the modifier
newValue := sw.getEnv(name)
switch modifier {
case '+':
if newValue != "" {
newValue = word
}
return newValue, nil
case '-':
if newValue == "" {
newValue = word
}
return newValue, nil
default:
return "", fmt.Errorf("Unsupported modifier (%c) in substitution: %s", modifier, sw.word)
}
}
return "", fmt.Errorf("Missing ':' in substitution: %s", sw.word)
}
// $xxx case
name := sw.processName()
if name == "" {
return "$", nil
}
return sw.getEnv(name), nil
}
func (sw *shellWord) processName() string {
// Read in a name (alphanumeric or _)
// If it starts with a numeric then just return $#
var name string
for sw.pos < len(sw.word) {
ch := sw.peek()
if len(name) == 0 && unicode.IsDigit(ch) {
ch = sw.next()
return string(ch)
}
if !unicode.IsLetter(ch) && !unicode.IsDigit(ch) && ch != '_' {
break
}
ch = sw.next()
name += string(ch)
}
return name
}
func (sw *shellWord) getEnv(name string) string {
for _, env := range sw.envs {
i := strings.Index(env, "=")
if i < 0 {
if name == env {
// Should probably never get here, but just in case treat
// it like "var" and "var=" are the same
return ""
}
continue
}
if name != env[:i] {
continue
}
return env[i+1:]
}
return ""
}