Master Linux I/O Redirection: stdout, stderr, and /dev/null Explained

Basic redirection syntax

$ command > file 2>&1
$ command >> file 2>&1

File descriptor numbers: 1 = stdout, 2 = stderr, 0 = stdin. The ampersand ( &) after a redirection operator tells the shell that the target is a descriptor, not a regular file. Thus 2>1 writes stderr to a file named 1, while 2>&1 sends stderr to the same destination as stdout.

Shorthand for both streams

The construct >file (or >&file) redirects both stdout and stderr to file, equivalent to >file 2>&1.

Order of redirections

Redirections are evaluated left‑to‑right. Example:

find /etc -name .bashrc > list 2>&1   # stdout → list, then stderr → same list
find /etc -name .bashrc 2>&1 > list   # stderr → original stdout, then stdout → list (stderr stays on original stdout)

Input redirection

command1 < file1

Discarding output

Redirect to /dev/null to discard data:

command > /dev/null          # discard stdout
command > /dev/null 2>&1    # discard both stdout and stderr

Common redirection patterns

Redirect stdout only: command > file Append stdout: command >> file Redirect stderr only: command 2> file Append stderr: command 2>> file Merge stderr into stdout: command > file 2>&1 or command >> file 2>&1 Redirect stdin and stdout:

command < in.txt > out.txt

Why merge stderr into stdout?

stderr is unbuffered while stdout is buffered; merging ensures the relative order of messages when both streams are captured.

Reference of redirection operators

command > file

– redirect stdout to

file

command < file

– redirect stdin from

file

command >> file

– append stdout to

file

n > file

– redirect descriptor n to

file

n >> file

– append descriptor n to

file

n >& m

– make descriptor n a copy of descriptor m (e.g., 2>&1) n <& m – duplicate input descriptor m onto

n

<< TAG

– here‑document: treat lines up to TAG as stdin