How to duplicate the contents of a string?

Question

linux

Content of file1.txt:
1
2
3
4
5
Output.txt should contain:
11
22
33
44
55

what command can duplicate the contents of a string in a string?

Answer 1

loops, echo and cat in the ass.
one command can replace everythingsed -r 's/(.*)/\1\1/' < ./file1.txt > ./output.txt

$ cat file1.txt
1
2
3
4
5
$ sed -r 's/(.*)/\1\1/' < ./file1.txt 
11
22
33
44
55

Answer 2

awk '{print $0$0}' file1.txt > output.txt

Answer 3

for s in $(cat file1.txt); do echo "$s$s" >> output.txt; done