Unix in a Nutshell, 4th Edition

Redirection Forms

File descriptor	Name	Common abbreviation	Typical default
0	Standard input	stdin	Keyboard
1	Standard output	stdout	Screen
2	Standard error	stderr	Screen

The usual input source or output destination can be changed with the redirection commands listed in the following sections.

Simple redirection

Command	Action
cmd > file	Send output of cmd to file (overwrite).
cmd >! file	Same as preceding, even if noclobber is set.
cmd >> file	Send output of cmd to file (append).
cmd >>! file	Same as preceding, even if noclobber is set.
cmd < file	Take input for cmd from file.
cmd << text	Read standard input up to a line identical to text (text can be stored in a shell variable). Input usually is typed at the keyboard or in the shell program. Commands that typically use this syntax include cat, echo, ex, and sed. If text is quoted (using any of the shell’s quoting mechanisms), the input is passed through verbatim. Otherwise, the shell performs variable and command substitutions on the input. When quoting text, the ending delimiter must be quoted identically.

Multiple redirection

Command	Action
cmd >& file	Send both standard output and standard error to file.
cmd >&! file	Same as preceding, even if noclobber is set.
cmd >>& file	Append standard output and standard error to end of file.
cmd >>&! file	Same as preceding, even if noclobber is set.
cmd1 |& cmd2	Pipe standard error together with standard output.
( cmd > f1 ...