bash: modifying a variable in a block

eantoranz · 06-28-2005, 10:12 AM

I have a very rare thing going on here. I have a repetitive cicle that modifies a variable. I actually do echos to check that the value changes. But once I finish the repetitive cicle, I check the variable and it's empty. Is there a simple explanation for this behavior?

Hko · 06-28-2005, 10:51 AM

Quote:

Is there a simple explanation for this behavior?

Yes, that's normal shell (bash) behaviour.
You could save it to a temp-file each time it changes. The after the loop ("circle") get it from the tmp-file,

eantoranz · 06-28-2005, 11:17 AM

Do I really need to go down to a file? :O

Hko · 06-28-2005, 11:28 AM

Hmm, now it actually tried it. And to my surprise the variable does keep the last value from the loop.

Code:

#!/bin/sh

TMP=1
while [ $TMP -lt 10 ]; do
        TMP=$(($TMP+1))  # change the var TMP
        echo In loop TMP = $TMP
done
echo At the end TMP = $TMP

If you loose the variable after the loop, what does your loop look like?

eantoranz · 06-28-2005, 11:52 AM

Well... It's a <i>litle</i> bit longer, you know.

Code:

let's create the route
grep '^[ \t]*[a-zA-Z]' /etc/dhep/interfaces | while read an_interface_line; do
        # Let's find the interface configuration file
        length=$(expr length "$an_interface_line")
        interfacestart=$(expr index "$an_interface_line" abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ)
        interfaceend=$(expr index "$an_interface_line" ' ')
        if [ $niterfaceend = 0 ]; then
                # Only the interface name is written
                interfacelength=$(($length-$interfacestart+1))
                interface=$(expr substr $an_interface_line $interfacestart $interfacelength)
                options=""
        else
                # Interface and options
                interface length=$(($interfaceend-$interfacestart))
                interface=$(expr substr "$an_interface_line" $interfacestart $interfacelength)
                optionslength=$(($length-$interfacelength+1))
                options=$(expr substr "$an_interface_line" $(($interfaceend+1)) $optionslength)
        fi
        echo $interface Options: $options

        # This interfaces configuration
        config=/etc/dhep/$interface.cfg
        if [ $(grep IP $config | wc -l) != 0 ]; then
                # This interface is configured for nexthop
                # let's find its IP address
                aline=$(cat $config | grep IP)
                ip=$(expr substr "$aline" 4 15)
                aline=$(cat $config | grep GW)
                gw=$(expr substr "$aline" 4 15)
                nexthop="nexthop via $gw dev $interface $options $nexthop"
                echo $nexthop > /etc/dhep/nexthop.tmp
        fi
done
echo nexthop: $nexthop

A little explanation is needed, (I guess).

This piece of script is run as part of a dhclient exit hook. This is used to change the router's multipath route in case a address changes.

The interfaces that are included ar in a file named /etc/dhep/interfaces. in each line of the file is the interface name and options (in case needed), for example: weight:

Code:

eth0
#eth2

Here, eth2 is not included for the nexthop evaluation.

And for each interface, the IP address and the GW provided by DHCP are written to a file called /etc/dhep/interface_name.cfg:

Code:

date Tue Jun 28 12:30:44 VET 2005
IP 201.249.120.235
GW 201.249.120.1

By the way... if you think there are better ways to program the script, I'm eager to get them.

Thanks!

(I tried the trick writing down the variable value to a file, and it works!, but I'd like NOT to do it.)