How to repeat each line of a file, from the Linux command line?

Question

How to make a file like:

one
two
three
...

into:

one one
two two 
three three
...

I'm thinking this must be possible with one simple Linux command. If my editor had block selections or macros I could do it easily. I'm using Geany right now. Maybe I need to switch editors again, or find a Geany plugin.

Answer 1

You need the paste command:

$ cat > file
one
two
three
four
...
$ paste file file
one     one
two     two
three   three
four    four
...     ...
$

Answer 2

In Bash, this reads from file.txt, and doubles every line, appending the output to out.txt

while read line; do echo "$line $line" >> out.txt; done < file.txt

Answer 3

$ cat t
ccc
ddd
aaa
bbb

$ perl -p -i -e 's/.*/$& $&/' t

$ cat t
ccc ccc
ddd ddd
aaa aaa
bbb bbb