Master Linux I/O Redirection: From Basics to Advanced Tricks

Output Redirection

In Linux shells, the > operator redirects standard output (fd 1) to a file, while 2>&1 redirects standard error (fd 2) to the same destination as stdout. The ampersand indicates that the target is a file descriptor, not a regular file.

1 => stdout
2 => stderr
0 => stdin

Thus 2>1 creates a regular file named “1” containing stderr, whereas 2>&1 sends stderr to the file descriptor 1 (typically /dev/stdout), effectively merging the two streams.

The combined form &>file or >&file is equivalent to >file 2>&1.

Order of Redirection

Redirections are processed left‑to‑right. For example:

find /etc -name .bashrc > list 2>&1   # stdout goes to list, then stderr is merged

If the order is reversed ( find /etc -name .bashrc 2>&1 > list), stderr is first merged into the original stdout stream, and the subsequent redirection of stdout to list does not affect the already‑merged stderr.

Input Redirection

command1 < file1

This feeds the contents of file1 to the command’s standard input.

Redirecting to /dev/null

Sending output to /dev/null discards it. To silence both stdout and stderr:

command > /dev/null 2>&1

Deeper Redirection

Every Unix process opens three standard file descriptors:

stdin (0) : default input source.

stdout (1) : default output destination.

stderr (2) : default error output.

Common redirection patterns: command > file – redirect stdout to file. command < file – redirect stdin from file. command >> file – append stdout to file. command 2> file – redirect stderr to file. command 2>> file – append stderr to file. command > file 2>&1 – merge stderr into stdout and write both to file. command < file1 > file2 – redirect stdin from file1 and stdout to file2.

Redirecting stderr to stdout is useful because stdout is usually buffered while stderr is not, allowing consistent ordering of output.