Some issues in examples/udhcp/simple.script

Rolf Eike Beer eb at emlix.com
Thu Sep 5 13:16:50 UTC 2019


Am Donnerstag, 5. September 2019, 14:57:15 CEST schrieb Denys Vlasenko:
> On Wed, Aug 14, 2019 at 9:50 AM Rolf Eike Beer <eb at emlix.com> wrote:
> > Hi all,
> > 
> > I have discovered some problems with examples/udhcp/simple.script. I know,
> > it's an example, and a simplified one. Most people will still just use it
> > because it works, so I would like to point out at least some things.
> > 
> > First, it does not really use ip. At least not if "command" does not
> > exist.
> 
> "command" is a mandatory shell builtin.
> 
> https://pubs.opengroup.org/onlinepubs/9699919799/utilities/command.html
> 
> https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html
> section "Command Search and Execution" specifically mentions it.
> 
> The reason for this mention is that its function
> can not reasonably be implemented as a separate external tool,
> since external tools don't know what functions are defined
> in current shell's invocation, thus "command FUNCNAME"
> won't work.
> 
> Since it's an internal builtin, using it should be a fastest way to test
> whether "ip" can be executed, without assuming a path
> ("test -x /usr/bin/ip") or forking a possibly external tool
> ("which ip").
> 
> Which shell do you use so that you don't have "command"?

Busybox ash:

# command ip
-sh: command: not found
# ip a
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
…

> > The difference between both cases is: ifconfig brings up the interface, ip
> > with this command does not. So if you are using ip this will just send
> > DHCP
> > request to a network interface that has link down, which will not get you
> > anywhere.
> 
> Yes, this is different. How about:
> 
>                 echo "Clearing IP addresses on $interface, upping it"
>                 if command -v ip >/dev/null; then
>                         ip addr flush dev $interface
>                         ip link set dev $interface up
>                 else
>                         ifconfig $interface 0.0.0.0
>                 fi

Which is basically what I'm using now, just different order of commands.

Thanks,

Eike
-- 
Rolf Eike Beer, emlix GmbH, http://www.emlix.com
Fon +49 551 30664-0, Fax +49 551 30664-11
Gothaer Platz 3, 37083 Göttingen, Germany
Sitz der Gesellschaft: Göttingen, Amtsgericht Göttingen HR B 3160
Geschäftsführung: Heike Jordan, Dr. Uwe Kracke – Ust-IdNr.: DE 205 198 055

emlix - smart embedded open source
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