Shell Programming and Scripting

hey guys,

i'm having some problems with my understanding of this whole regexp thing.

I'm just exploring here really by trying to do various match & filter & print stuff on the console.

I figured i want to get the IP of an interface. So my idea here was that first i filter to extract only the line with "inet" from ifconfig. Hence i type :

ifconfig rl0 | grep inet

this yields the two lines of ipv4 and ipv6 address configuration. Then i only take the 2nd field of each line since that's where the address really is hence i type :

ifconfig rl0 | grep inet | awk '{print $2}'

and i end up with both the ipv4 and ipv6 address so that's where it gets tricky for me. I thought that i'd put a last filter with a regexp to get the line that begins with a word made up of at least 1 and at most 3 digits:

ifconfig rl0 | grep inet | awk '{print $2}' | grep '^[0-9]{1,3}'

and it doesn't work, ie i end up with a void set. However if i remove the {1,3} quantifier, it does work and i get the ipv4 address !

I don't understand this at all. Why does it work with only one occurence but if i say at least 1 and at most 3 it just fails !!! Is there something flawed in my understanding of regexps ? Can someone provide some insights ?

PS:
i don't think it matters but just in case, i'm using tcsh

It depends on whether the version of grep you are using understands extended regex.

Meybe this would work better?

Code:

ifconfig rl0 |  awk '/inet / {print $2}'

Try something like this:

Code:

ifconfig rl0 | gawk --re-interval '/^[0-9]{1,3} .*inet/ {print $2}'

If you're using awk, you seldom need to use grep.

Use egrep if you have it indstalled ( hint: which egrep)

ifconfig | awk '/inet/{ print $2}' | egrep "[0-9]{1,3}"

Thanks guys for your help.

I should've mentionned earlier that i was more interested in understanding why my command didn't work than in how actually getting the IP.

So anyway Reborg you were right it has to do with grep only understanding ultra basic regexp and as Vish_indian said, egrep is the way to go in that case.

So by using egrep it just works fine !

PS:
i need to work more on awk i guess ...