docker command line list arguments - go

Question

When I am starting the docker daemon I am modifying the dns server so that the containers have a modified /etc/resolv.conf. Looking at the usage message I see:
$ docker --help
Usage: docker [OPTIONS] COMMAND [arg...]
A self-sufficient runtime for linux containers.
Options:
--api-enable-cors=false Enable CORS headers in the remote API
-b, --bridge="" Attach containers to a prexisting network bridge
use 'none' to disable container networking
--bip="" Use this CIDR notation address for the network bridge's IP, not compatible with -b
-D, --debug=false Enable debug mode
-d, --daemon=false Enable daemon mode
--dns=[] Force Docker to use specific DNS servers
--dns-search=[] Force Docker to use specific DNS search domains
-e, --exec-driver="native" Force the Docker runtime to use a specific exec driver
... etc ...
The --dns is what I want to pass, it shows a 'list' with the [], which after much trial and error I finally got this to work:
--dns 127.0.0.1 --dns 8.8.8.8
which deposits :
nameserver 127.0.0.1
nameserver 8.8.8.8
in to the /etc/resolv.conf file.
Is this the correct way to provide a list to docker (and presumably any go) program?

Answer 1

This is a way of passing multiple arguments to a program in Go but certainly not the only way. This is accomplished by defining a type that implements the Value interface. The flag package on flag.Parse() iterates though the argument list matching the name to a registered Value and calling the Set(string) function on the Value. You can use this to append each value of a given name to a slice.
type numList []int
func (l *numList) String() string {
return "[]"
}
func (l *numList) Set(value string) error {
number, err := strconv.Atoi(value)
if err != nil {
return fmt.Errorf("Unable to parse number from value \"%s\"", value)
}
*l = append(*l, number)
return nil
}
This new type can be registered as a flag variable. In the following example the application takes n num command line arguments that are converted to an integer and added to a list.
var numbers numList
func main() {
flag.Var(&numbers, "num", "A number to add to the summation"
flag.Parse()
sum := 0
for _, num := range numbers {
sum += num
}
fmt.Printf("The sum of your flag arguments is %d.\n", sum)
}
This could have been easily done with a string flag and have users pass a delimited list. There is no established convention within the Go language and each application can provide whatever implementation of best fit.