Like the cat command, the tee command copies standard input to standard output unaltered. Simultaneously, however, it also copies that same standard input to one or more files. tee is most often found in the middle of pipelines, writing some intermediate data to a file while also passing it to the next command in the pipeline:

$ who | tee original_who | sort

In this command line, tee writes the output of who to the file original_who, and then passes along that same output to the rest of the pipeline (sort), producing sorted output on screen.

Useful options