xargs로 여러 줄을 한줄로 줄이기

https://stackoverflow.com/questions/15758814/turning-multiple-lines-into-one-line-with-comma-separated-perl-sed-awk

aaa
bbb
ccc
ddd

cat file | xargs

aaa bbb ccc ddd 

cat file | xargs | sed -e 's/ /,/g'

aaa,bbb,ccc,ddd 

변태 같은 명령어라고 표현해야 하나..