(sed '' file1 | sort -o file2)


what is (sed '' file1) means?

cat file1

sort file1 > file2

Thanks for the response. What exactly single quotes meaning in below command?

sed '' file1

They mean what they always mean in a sh script. It's an empty string. sed never sees those quotes, but it is passed an empty string as its first argument. sed interprets this as an empty list of commands. Given no commands, and without the -n option, sed just prints each line it reads. In other words, as Scrutinizer mentioned, it behaves like cat.

Regards,
Alister