group lines according to first word How can I modify the following content of a file: cat:persian/young-1 cat:winter/young-2 cat:summer/wild-3 dog:persian/young-1 dog:winter/young-2 dog:summer/wild-3 To : cat:persian/young-1 cat:winter/young-2 cat:summer/wild-3 dog:persian/young-1 dog:winter/young-2 dog:summer/wild-3 It's not specific to dog or cat, it's more of symbolic representation of whatever the first word/term is

You could do something like:


awk -F: 'NR>1 && $1 "" != last {print ""}; {print; last = $1}'


The `""` is to force string comparison. Without it, it wouldn't work properly in input like:


100:foo
100:bar
1e2:baz
1e2:biz


Where `100` and `1e2` would be compared as numbers.