shell scripting

[Marius Gedminas]

On Sat, Nov 09, 2013 at 05:23:19PM +0100, pluto mars wrote:
> I need some help with shell scripting (/bin/sh) in ubuntu.
> 
> In a shell script I want to turn down an interface, say eth0 and then turn
> it up.

If you told us why you want to write this shell script, we might be able
to come up with a better way to solve whatever problem you're trying to
solve.

> So something like
> 
> ifdown eth0
> ifup eth0
> 
> But what I want in my shell script is to check and wait till eth0 is up and
> then start doing something next in my shell script.
> So something like "if ( eth0 not is up ) { sleep 1 }. But how do I do this
> in shell scripting ?

Do you use DHCP?  If so, it may not be sufficient to wait for the
interface to be up, you probably also want for a valid DHCP lease.

The simplest way I can think of is to wait until the default route gets
configured:

    while ! ip r | grep -q default; do
        sleep 1
    done

> And then also I want something similar for checking if syslog process is up
> or not. Because actually I am writing syslog through that interface to
> another computer. So I want to check if eth0 is up. If up, check if syslog
> process is up, if not start, if yes, write something to syslog.

Why would syslog not be up?  Upstart ensures rsyslog will be present via
/etc/init/rsyslog.conf.

You can check for its presence and with something like

    if [ -z "$(pidof -s rsyslogd)" ]; then
        service rsyslogd start
    fi

or just issue the 'service rsyslogd start' unconditionally, it does
nothing if rsyslogd is already running.

Marius Gedminas
-- 
Gates' Law: Every 18 months, the speed of software halves.
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