Unix: confusing use of the Tee -command

Question

Manual states that the tee is a \"pipe fitting\"-tool. The cases [1] confuse me:

1. case

echo \"foo bar\" | sudo tee -a /path/to/some/fi         


        

Answer 1

tee is normally used to split the output of a program so that it can be both displayed and saved in a file. The command can be used to capture intermediate output before the data is altered by another command or program. The tee command reads standard input, then writes its content to standard output. It simultaneously copies the result into the specified file(s) or variables

tee [OPTION]... [FILE]...

For instance

tee [ -a ] [ -i ]... [ File ]...

• -a Appends the output to the end of File instead of writing over it.

• -i Ignores interrupts.

With sudo and appending to the file with your example in the question

ls -l | sudo tee -a file.txt