> sends output to a file, and you see nothing. Sometimes you want both: see the output and save it. tee does this.

tee writes each line it reads to the file and to its standard output:

learner@tidepool:~$ grep WARN shop.log | tee warnings.txt
12:20 WARN low stock: cake
learner@tidepool:~$ cat warnings.txt
12:20 WARN low stock: cake

In the middle of a pipeline

Because tee writes to standard output, the pipeline can continue. Save the lines, then count them:

learner@tidepool:~$ grep ERROR shop.log | tee errors.txt | wc -l
2

Add to the file: -a