Why does cut -d ' ' not work with space in this case?

Question

I've no issue with this cut command.

wolf@linux:~$ echo ab cd ef
ab cd ef

wolf@linux:~$ echo ab cd ef | cut -d ' ' -f 1
ab

wolf@linux:~$ echo ab cd ef | cut -d ' ' -f 2
cd

However, when I try the same command with different input like this, I did not get the output as expected.

wolf@linux:~$ ip address show eth0 | grep 'inet '
    inet 10.10.10.10/24 brd 10.10.10.255 scope global dynamic eth0

wolf@linux:~$ ip address show eth0 | grep 'inet ' | cut -d ' ' -f 1

wolf@linux:~$ ip address show eth0 | grep 'inet ' | cut -d ' ' -f 2

What's wrong in the second example?

awk doesn't seems to have a problem with the same input, strange.

wolf@linux:~$ ip address show eth0 | awk '/inet / {print $1}'
inet

wolf@linux:~$ ip address show eth0 | awk '/inet / {print $2}'
10.10.10.10/24

Answer 1

cut takes each and every delimiter as meaningful, even if there are consecutive spaces. This is unlike awk, which by default splits on whitespace, but takes multiple whitespace as only one delimiter, and ignore leading whitespace.

You could get awk to behave similarly by setting the field separator FS to [ ]:

$ ip add show lo | awk -F'[ ]' '$5 == "inet" {print $6}'
127.0.0.1/8

That has to be set like that, as a regexp, since a single space by itself in FS marks the default, special operation.

See, e.g. the GNU awk manual has to say on field separators.

Answer 2

The line you grep from the output starts with 4 spaces, if you select the fifth field you'll get the first string:

admin:~ # ip address show eth0 | grep 'inet ' | cut -d " " -f 5
inet

Answer 3

I've created a patch that adds new -m command-line option to cut, which works in the field mode and treats multiple consecutive delimiters as a single delimiter. This basically solves the OP's question in a rather efficient way. I also submitted this patch upstream a couple of days ago, and let's hope that it will be merged into the coreutils project.

There are some further thoughts about adding even more whitespace-related features to cut, and having some feedback about all that would be great. I'm willing to implement more patches for cut and submit them upstream, which would make this utility more versatile and more usable in various real-world scenarios.