Special char or wildcard in awk field separator

awkshell-scripting

Would appreciate to know about the special char or wildcard in the following command:

/sbin/ifconfig eth0 | awk -F ' *|:' '/inet addr/{print $4}'

What does the code below mean?

awk -F ' *|:' '

Best Answer

The -F flag (for awk) means to use the following extended regular expression as a Field separator Which means it treats any of the characters between the single quotes here: ' *|:' as delimiters.

Awk will then print the 4th field that has a match with inet addr.

For example: if this were the output for ifconfig eth0: 

eth0      Link encap:Ethernet  HWaddr 09:00:12:90:e3:e5  
          inet addr:192.168.1.29 Bcast:192.168.1.255  Mask:255.255.255.0
          inet6 addr: fe80::a00:27ff:fe70:e3f5/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:54071 errors:1 dropped:0 overruns:0 frame:0
          TX packets:48515 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000 
          RX bytes:22009423 (20.9 MiB)  TX bytes:25690847 (24.5 MiB)
          Interrupt:10 Base address:0xd020

It should print out 

192.168.1.29

To break down the regular expression:

' *|:'

the 

' *'

Means any number of spaces, and the remaining characters have no special meaning, so either of them can be a delimiter. The * itself is not a delimiter, it just means any number of the preceding character, a space.

Related Topic