有没有一种方法可以确定在使用`flag.VisitAll`时是否设置了标志？

Question

I'm using go's native "flag" package.

Built into it is the ability to visit all currently defined flags, using flag.VisitAll.

I'm trying to build a snippet that tries to fetch the value for that flag from an environment variable if one exists and in-case the flag was not set, and I can't find a way to determine whether a specific flag was set or not.

Is there any way to achieve that without implementing new parameter types?

Answer 1

There is no function to walk over the unset command line flags. This functionality can be implemented, however, by taking the difference between the flags returned by VisitAll and Visit; the former walks over all flags, while the latter walks over set flags:

func UnsetFlags(fs *flag.FlagSet) []*flag.Flag {
    var unset []*flag.Flag
    fs.VisitAll(func(f *flag.Flag) {
        unset = append(unset, f)
    })
    fs.Visit(func(f *flag.Flag) {
        for i, h := range unset {
            if f == h {
                unset = append(unset[:i], unset[i+1:]...)
            }
        }
    })
    return unset
}

You can use that function after your flag.Parse call to set any unset flags to their environment value:

for _, f := range UnsetFlags(flag.CommandLine) {
    v := os.Getenv(f.Name)
    f.Value.Set(v)
}