Copy command output data to standard output and to a file

Use the tee command: 

$ [COMMAND] | tee [OUTPUT.FILE]

Where:

[COMMAND] is the command you want to execute.

[OUTPUT.FILE] is the file you want to write in.

Variation for writing the standard error output:

$ [COMMAND] 2>&1 | tee [OUTPUT.FILE]

Where:

[COMMAND] is the command you want to execute. 

[OUTPUT.FILE] is the file you want to write in.

2>&1 redirects stderr(2) to stdout(1).

Notes:

2>&1 is different that 2>1.

2>1 means writing to a file with name 1 and 2>&1 means write to the standard output.