Displaying IP address on eth0 interface

Awk

ifconfig eth0 | awk '/inet addr/{split($2,a,":"); print a[2]}'

split function in the above awk command splits the second column based on the delimiter : and stores the splitted value into an associative array a. So a[2] holds the value of the second part.

sed

ifconfig eth0 | sed -n '/inet addr/s/.*inet addr: *\([^[:space:]]\+\).*/\1/p'

In basic sed , \(...\) called capturing group which is used to capture the characters. We could refer those captured characters through back-referencing. \([^[:space:]]\+\) captures any character but not space one or more times.

grep

ifconfig eth0 | grep -oP 'inet addr:\K\S+'

\K discards the previously matched characters from printing at the final and \S+ matches one or more non-space characters.

Perl

ifconfig eth0 | perl -lane 'print $1 if /inet addr:(\S+)/'

One or more non-space characters which are next to the inet addr: string are captured and finally we print those captured characters only.