The following command displays output only on the screen (stdout).

$ ls 

The following command writes the output only to the file and not to the screen.

$ ls > file

The following command (with the help of tee command) writes the output both to the screen (stdout) and to the file.

$ ls | tee file

You can also use tee command to store the output of a command in a file and redirect the same output to another command.

The following command will write current crontab entries to a file crontab-backup.txt and pass the crontab entries to sed command, which will do the substituion. After the substitution, it will be added as a new cron job.

$ crontab -l | tee crontab-backup.txt | sed 's/old/new/' | crontab –

You can pipe your output to multiple files (including your terminal) by using tee like this:

$ ls | tee file1 file2 file3

By default tee command overwrites the file. You can instruct tee to append to the file using the –a option as shown below.

$ ls | tee –a file