Go CLI Intro
flag
命令行flag解析
flag 长短选项
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16func main() {
var name string
// -flag: 仅支持布尔类型
// -flag x: 仅支持非布尔类型
// -flag=x: 均支持
// 长短选项 分开两次调用
flag.StringVar(&name, "name", "Golang flag tour", "help")
flag.StringVar(&name, "n", "Golang flag tour", "help")
flag.Parse()
log.Printf("name: %s", name)
}
// ➜ go run flag/demo.go -name='hello world' -n 'hello world'
// 2021/09/11 18:26:39 name: hello worldflag 子命令
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32func main() {
var name string
flag.Parse()
args := flag.Args()
if len(args) <= 0 {
return
}
switch args[0] {
case "go":
goCmd := flag.NewFlagSet("go", flag.ExitOnError)
goCmd.StringVar(&name, "name", "Golang language", "help")
_ = goCmd.Parse(args[1:])
case "py":
pyCmd := flag.NewFlagSet("py", flag.ExitOnError)
pyCmd.StringVar(&name, "name", "Python language", "help")
_ = pyCmd.Parse(args[1:])
}
log.Printf("name: %s", name)
}
// ➜ go run flag/flag_sub_cli.go go -name=golang
// 2021/09/11 18:39:47 name: golang
//
// ➜ go run flag/flag_sub_cli.go go -n=python
// flag provided but not defined: -n
// Usage of go:
// -name string
// help (default "Golang language")
// exit status 2flag 源码分析
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121# flag.Parse
// CommandLine is the default set of command-line flags, parsed from os.Args.
// The top-level functions such as BoolVar, Arg, and so on are wrappers for the
// methods of CommandLine.
var CommandLine = NewFlagSet(os.Args[0], ExitOnError)
// Parse parses the command-line flags from os.Args[1:]. Must be called
// after all flags are defined and before flags are accessed by the program.
func Parse() {
// Ignore errors; CommandLine is set for ExitOnError.
CommandLine.Parse(os.Args[1:])
}
# FlagSet.Parse
> parse异常分流处理
// Parse parses flag definitions from the argument list, which should not
// include the command name. Must be called after all flags in the FlagSet
// are defined and before flags are accessed by the program.
// The return value will be ErrHelp if -help or -h were set but not defined.
func (f *FlagSet) Parse(arguments []string) error {
f.parsed = true
f.args = arguments
for {
seen, err := f.parseOne()
if seen {
continue
}
if err == nil {
break
}
switch f.errorHandling {
case ContinueOnError:
return err
case ExitOnError:
if err == ErrHelp {
os.Exit(0)
}
os.Exit(2)
case PanicOnError:
panic(err)
}
}
return nil
}
# FlagSet.parseOne
// parseOne parses one flag. It reports whether a flag was seen.
func (f *FlagSet) parseOne() (bool, error) {
if len(f.args) == 0 {
return false, nil
}
s := f.args[0]
if len(s) < 2 || s[0] != '-' {
return false, nil
}
numMinuses := 1
if s[1] == '-' {
numMinuses++
if len(s) == 2 { // "--" terminates the flags
f.args = f.args[1:]
return false, nil
}
}
name := s[numMinuses:]
if len(name) == 0 || name[0] == '-' || name[0] == '=' {
return false, f.failf("bad flag syntax: %s", s)
}
// it's a flag. does it have an argument?
f.args = f.args[1:]
hasValue := false
value := ""
for i := 1; i < len(name); i++ { // equals cannot be first
if name[i] == '=' {
value = name[i+1:]
hasValue = true
name = name[0:i]
break
}
}
m := f.formal
flag, alreadythere := m[name] // BUG
if !alreadythere {
if name == "help" || name == "h" { // special case for nice help message.
f.usage()
return false, ErrHelp
}
return false, f.failf("flag provided but not defined: -%s", name)
}
if fv, ok := flag.Value.(boolFlag); ok && fv.IsBoolFlag() { // special case: doesn't need an arg
if hasValue {
if err := fv.Set(value); err != nil {
return false, f.failf("invalid boolean value %q for -%s: %v", value, name, err)
}
} else {
if err := fv.Set("true"); err != nil {
return false, f.failf("invalid boolean flag %s: %v", name, err)
}
}
} else {
// It must have a value, which might be the next argument.
if !hasValue && len(f.args) > 0 {
// value is the next arg
hasValue = true
value, f.args = f.args[0], f.args[1:]
}
if !hasValue {
return false, f.failf("flag needs an argument: -%s", name)
}
if err := flag.Value.Set(value); err != nil {
return false, f.failf("invalid value %q for flag -%s: %v", value, name, err)
}
}
if f.actual == nil {
f.actual = make(map[string]*Flag)
}
f.actual[name] = flag
return true, nil
}