Lesson 6.4: Redirecting input and output

The three standard streams and their numbers, writing output to files with > and >>, reading input with <, separating errors with 2>, joining them with 2>&1, throwing output away into /dev/null, and tee.

16 min

You will learn to

Name stdin, stdout and stderr and their numbers 0, 1, 2
Write and append to files with > and >>
Feed a file to a command with <
Send errors elsewhere with 2> and combine streams with 2>&1
Discard output with /dev/null and copy it with tee

Key terms

Standard streams: Every program starts with three open channels: standard input (stdin, 0), standard output (stdout, 1) and standard error (stderr, 2). By default stdin is the keyboard and the other two are the terminal.
> and >>: > redirects stdout into a file, creating it or overwriting it; >> appends to the end.
<: Redirects stdin to read from a file.
2> and 2>&1: 2> redirects stderr; 2>&1 sends stderr to wherever stdout currently goes. &> file is bash's shortcut for > file 2>&1.
/dev/null: A device that discards everything written to it and returns nothing when read.
tee: Copies its input to one or more files and to its output, like a T-shaped pipe fitting.

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