Standard file streams

When commands are executed, by default there are three standard file streams (or descriptors) always open for use: standard input (standard in or stdin), standard output (standard out or stdout) and standard error (or stderr).

stdin is normally the keyboard, stdout the terminal and stderr to a log file

In Linux, all open files are represented internally by what are called file descriptors. Simply put, these are represented by numbers starting at zero. stdin is file descriptor 0, stdout is file descriptor 1, and stderr is file descriptor 2.

IO Redirection

These streams can be redirected to:

• Read input from a file instead of the keyboard
• Output to a file instead of the screen
• Redirect the error messages

To redirect the input

command < input_file

For the output (and the standard error), > can be used, in combination with the file descriptor:

command > output_file # Redirects stdout
command 1> output_file #Same but with file descriptor
command 2> log_file # Redirects stderr
command > all_output_file 2>&1 #Both stdout and stderr to the same file
command >& all_output_file #Same as above, only in BASH