diff -w (or -b, -Z, -E):

(Note: -Z / --ignore-trailing-space seems to be a relative new option)

/tmp>( echo '  Hello               World  !' ; echo 'blah' ) > 1.txt

/tmp>( echo 'Hello World !      ' ; echo 'foo' ) > 2.txt

/tmp>diff 1.txt 2.txt
1,2c1,2
<   Hello               World  !
< blah
---
> Hello World !
> foo

/tmp>diff -w 1.txt 2.txt
2c2
< blah
---
> foo

/tmp>
[download]

Alexander