Am Donnerstag, 5. September 2019, 14:57:15 CEST schrieb Denys Vlasenko: > On Wed, Aug 14, 2019 at 9:50 AM Rolf Eike Beer <[email protected]> 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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