Repeat each line using sed

Question

I want to duplicate each line of the input using sed. Ie when the input looks like:

first
secod
third
fourth
...

I want the output to look like:

first first
second second
third third
fourth fourth
... ...

and so on.

If this is impossible using sed, then what do you recommend? I prefer the most simple solution.

Answer 1

这似乎是paste命令的定制工作：

paste -d " " file file

Answer 2

One way:

$ sed 's/.*/& &/' file
first first
second second
third third
fourth fourth

Answer 3

Use awk,

awk '{print $1, $1}' file

or as commented, you can do the following in case the words contain white spaces:

awk '{print $0, $0}' file