Tee

With tee it is possible to read from standard input and write to standard output and files (or commands) at the same time.

Usage

Log into file and stdout: foo | tee output.file

Append to a file: foo | tee -a output.file

Include stderr: foo 2>&1 | tee output.file

2>&1 redirects channel 2 (stderr/standard error) into channel 1 (stdout/standard output), such that both is written as stdout