Piping and Redirection

There are STDIN(0), STDOUT(1) & STDERR(2):

Stream Name	Description
Standard Input (STDIN)	Data fed into the program
Standard Output (STDOUT)	Output from the program (defaults to terminal)
Standard Error (STDERR)	Error messages (defaults to terminal)

We can use |, < or > to pass any of the 3 streams to other programs:

Redirecting to a New File

• echo "test" > file.txt
  • if the file already exists it is overwritten!

Redirecting to an Existing File

• to append to a file use >>
  • echo "test" >> file.txt

Redirecting from a File

• echo < file.txt

Redirecting STDERR

• ls ./none-existent-file.txt 2>error.txt

Piping

• cat error.txt | wc -m

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Relevant Note(s): Linux Basics