Duplicate lines beginning with “x”, replacing “x” in the duplicate with “y”?

Question

I have a file with the following pattern:

foo 111
baz
foo 222
baz
foo 333
baz

which needs to be turned into

foo 111
bar 111
baz
foo 222
bar 222
baz
foo 333
bar 333
baz

So, duplicate all the lines beginning with foo, and change only foo in the duplicate but leave the rest of the line (which differs in each case) intact. How to do this?

Answer 1

Here you go:

:g/foo/t.|s//bar

Decomposing:

:g/foo/    " start a global command applied on all lines matching 'foo'
t.         " duplicate the current line (the cursor is now on the new line)
|          " chain a new command
s//bar     " substitute the last searched element with 'bar'

Because the g command will update the search pattern, so you can omit the pattern to replace in the substitute command. (ref: :h :g, search for search pattern).

---

Older version:

:g/foo/norm! yyp:s/foo/bar^M

Decomposing:

:g            start a global command
/foo/         apply only on lines having 'foo'
norm!         execute a normal command
yyp           duplicate the line
:s//bar       replace foo with bar on the line (the duplicated one)
^M            add enter to execute the substitution

To insert the ^M press Ctrl+v and enter.

Note: I originally came up with the "older" version, before I learned about the t command. I'll leave it but I won't recommend using it. The first one is cleaner more straightforward.

Answer 2

I do stuff like this all the time and just do something like %s/^foo \(.*\)/foo \1\rbar \1/ (partially because I also do a lot of similar tasks that can't be turned into something clever using a copy function, and \(.*\) is part of my muscle memory by now).

I suspect it's not POSIX-compliant (it doesn't work on other vi clones), but using ^M (C-VReturn) instead of \r seems to work on most other vi clones.