golang to parse command line flags/switches/arguments

[Tong Sun]

Hi, 

I'm trying to learn the flag package that implements command-line flag parsing, because its doc is not too clear to me. Here are my questions:

1. The two examples I found, 

https://github.com/DaddyOh/golang-samples/blob/master/flag.go

https://groups.google.com/forum/#!msg/golang-nuts/qZXEmTQ5Kyc/o0Uo_eQr9YwJ

(and https://github.com/lanep/golang-me/blob/master/misc_examples/flag.go)

both contain a function "usage()", but there is no explicit calling of function "usage()" in the code (only a hint that it will somehow get called), nor the function "usage()" is in the flag package doc. 

2. I'm trying to compare it with the traditional *nix getopt. And seems that getopt will rearrange command line arguments so that switches comes first, while the flag package will not, correct? Also, with getopt, long options may be  abbreviated, as long as the abbreviation is not ambiguous. Is it true for the flag package's behavior?

3. Seems that there is a hidden/undocumented flag -help (http://blog.brandonc.me/2013/04/golang-flag-help-h.html). Why is it undocumented?

Now you can see why I'm scratching my head for this simple package. Again, I'm trying to learn the flag package by example, so could you give me a short example that, 

- have one mandatory switch and an optional one

- will prompt the usage when 

 1. no command line arguments

 2. the mandatory switch is missing

- will prompt the usage when wrong switches are provided

- also throw in handling the additional non-flag arguments (via flag.Args()) please

TIA

[Volker Dobler]

Am Sonntag, 22. September 2013 21:48:35 UTC+2 schrieb Tong Sun:

1. The two examples I found, 

[...]

both contain a function "usage()", but there is no explicit calling of function "usage()" in the code (only a hint that it will somehow get called), nor the function "usage()" is in the flag package doc. 

See var Usage under http://golang.org/pkg/flag/#pkg-variables

 

2. I'm trying to compare it with the traditional *nix getopt. And seems that getopt will rearrange command line arguments so that switches comes first, while the flag package will not, correct? Also, with getopt, long options may be  abbreviated, as long as the abbreviation is not ambiguous. Is it true for the flag package's behavior?

No, you may not abbreviate anything here.

 

3. Seems that there is a hidden/undocumented flag -help (http://blog.brandonc.me/2013/04/golang-flag-help-h.html). Why is it undocumented?

Again see http://golang.org/pkg/flag/#pkg-variables and http://golang.org/pkg/flag/#FlagSet.Parse

 

Now you can see why I'm scratching my head for this simple package. Again, I'm trying to learn the flag package by example, so could you give me a short example that, 

The example here http://golang.org/pkg/flag/#example_ show what you can do. 

 

- have one mandatory switch and an optional one

There are no mandatory flags in Go.

 

- will prompt the usage when 

 1. no command line arguments

 2. the mandatory switch is missing

Package flag cannot be parametrized to do so: You must write some code yourself.

 

- will prompt the usage when wrong switches are provided

That is the default.

 

- also throw in handling the additional non-flag arguments (via flag.Args()) please

Cannot be done without code. 

V. 

[Tong Sun]

On Sun, Sep 22, 2013 at 4:23 PM, Volker Dobler <dr.volke...@gmail.com> wrote:

1. The two examples I found, 

[...]

both contain a function "usage()", but there is no explicit calling of function "usage()" in the code (only a hint that it will somehow get called), nor the function "usage()" is in the flag package doc. 

See var Usage under http://golang.org/pkg/flag/#pkg-variables

I'm too new to GO to understand what you want me to see. Are you saying that there is a hidden link between var Usage and function "usage()"?

 

3. Seems that there is a hidden/undocumented flag -help (http://blog.brandonc.me/2013/04/golang-flag-help-h.html). Why is it undocumented?

Again see http://golang.org/pkg/flag/#pkg-variables and http://golang.org/pkg/flag/#FlagSet.Parse

Again, I'm too new to GO to understand what you want me to see. I've read them before I posted my questions. 

Thanks for your help though. 

[Tong Sun]

On Sun, Sep 22, 2013 at 4:23 PM, Volker Dobler <dr.volke...@gmail.com> wrote:

Am Sonntag, 22. September 2013 21:48:35 UTC+2 schrieb Tong Sun:

1. The two examples I found, 

[...]

both contain a function "usage()", but there is no explicit calling of function "usage()" in the code (only a hint that it will somehow get called), nor the function "usage()" is in the flag package doc. 

Most probably this usage gets called somehow manually in the samples you found.

No, I double checked again, 

https://github.com/lanep/golang-me/blob/master/misc_examples/flag.go

https://groups.google.com/forum/#!msg/golang-nuts/qZXEmTQ5Kyc/o0Uo_eQr9YwJ

As I said, there is no explicit calling of function "usage()" in the code. 

[Matt Harden]

The first example does call usage(). I haven't checked the second.

https://github.com/lanep/golang-me/blob/master/misc_examples/flag.go#L24

[Ian Lance Taylor]

On Sun, Sep 22, 2013 at 12:48 PM, Tong Sun <sunto...@gmail.com> wrote:
>
> 2. I'm trying to compare it with the traditional *nix getopt. And seems that
> getopt will rearrange command line arguments so that switches comes first,
> while the flag package will not, correct? Also, with getopt, long options
> may be abbreviated, as long as the abbreviation is not ambiguous. Is it
> true for the flag package's behavior?

For what it's worth, the behaviour you are describing is not the
traditional *nix getopt. It's GNU getopt. It was GNU getopt that
introduced rearranging flags to the front and permitting
abbreviations. The Go flag package is closer to the traditional
getopt.

Ian

[Gustavo Niemeyer]

FWIW, Roger Peppe published a fork of the standard flag package a
while ago to behave more like the GNU flag parsing, so juju would stay
backwards compatible:

http://godoc.org/launchpad.net/gnuflag

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gustavo @ http://niemeyer.net