Getting a substring in bash script - linux

Question

I was trying to get a substring in a bash script however the way I did is not a good solution. I'm parsing the response of "ifconfig" command and trying to get the first network interface name.
result of ifconfig:
eth0 Link encap:Ethernet HWaddr b8:27:eb:6d:a1:92
UP BROADCAST MULTICAST MTU:1500 Metric:1
RX packets:0 errors:0 dropped:0 overruns:0 frame:0
TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:0 (0.0 B) TX bytes:0 (0.0 B)
lo Link encap:Local Loopback
inet addr:127.0.0.1 Mask:255.0.0.0
UP LOOPBACK RUNNING MTU:65536 Metric:1
RX packets:73 errors:0 dropped:0 overruns:0 frame:0
TX packets:73 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
RX bytes:7099 (6.9 KiB) TX bytes:7099 (6.9 KiB)
wlan0 Link encap:UNSPEC HWaddr ****
UP BROADCAST RUNNING PROMISC MULTICAST MTU:1500 Metric:1
RX packets:792698 errors:0 dropped:792552 overruns:0 frame:0
TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:219274179 (209.1 MiB) TX bytes:0 (0.0 B)
wlan5 Link encap:Ethernet HWaddr ****
inet addr:**** Bcast:**** Mask:****
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:48934 errors:0 dropped:3422 overruns:0 frame:0
TX packets:21217 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:14458518 (13.7 MiB) TX bytes:2692948 (2.5 MiB)
getting first interface name which is wlan0
conf=`ifconfig`
net=${conf:670:6}
I didn't understand but position sometimes changes this is why i can't use index 670. Wlan0 can be wlan1,wlan2 and so on... I can't specifically search for wlan0. Any suggestions?

Answer 1

GNU awk
ifconfig | awk -vRS= '!/^(eth0|lo)/{print $1;exit}'
Skips etho and lo blocks and prints the first field of the next, then quits.

Answer 2

At first I thought the question was to get all the wlans and answered this:
$ ifconfig | egrep -A6 '^wlan[0-9]:'
but then it was pointed out that this was GNU, not BSD so I should have said
$ ifconfig | egrep -A6 '^wlan[0-9]'
(no colons). Then it was clarified that only the first one was needed, so perhaps
$ ifconfig | head -n 6
is a better answer?
If there are not exactly 6 lines in any description, then this isn't a very good answer.
Another approach is this:
$ ifconfig eth0 || ifconfig lo0 || ifconfig wlan0 || ifconfig wlan1
and so on. The || means if the first part fails, try the second. You'll get error messages until you hit one that works.
Now here is something better!
$ ifconfig | head -n 1
will give you the first one. Take that line, cut out the first thing on the line, then pass that to ifconfig. This should work on Linux:
$ ifconfig | head -n 1 | awk '{print $1}' | xargs ifconfig

Answer 3

What about something like?
ifconfig | grep -v "^ " "eth0" "lo" | head -1 | cut -c1-6
Have not a Linux pc on which to test it, though.
Basically, i just extract all the lines which do have the name of a net interface, removing all the eth0 and lo stuff, then I get the first one and get only the chars I need