Redirection and Stream
Real World Example
Log in to the server using SSH, say if we want to find all log files in the server, we can use the following command:
find / -name '*.log' > /path/to/logfile.log
This way, all the output of find / -name '*.log' will be redirect to logfile.log file.
However, the certain logged in user may not have the permission to access all the files, using above command will result in putting not only the accessible files but also the error message to the logfile.log. To avoid it, we can use
find / -name '*.log' 2> /dev/null
2> /dev/null is used to redirect the error message to /dev/null, which means the error message will not be displayed.
Linux Stream
- stander input（stdin）： 0, usually input from the keyboard.
- stander output（stdout）： 1, usually output to the terminal, used to output normal information.
- stander error（stderr）： 2, usually output to the terminal, used to output error information.
About /dev/null
/dev/null is a special file in Unix-like operating systems that discards all data written to it but reports that the write operation succeeded. It provides essentially the same functionality as a black hole on a filesystem.
Redirection and &1 and &2
File descriptor 1 is the standard output (stdout). File descriptor 2 is the standard error (stderr).
At first, 2>1 may look like a good way to redirect stderr to stdout. However, it will actually be interpreted as "redirect stderr to a file named 1".
& indicates that what follows and precedes is a file descriptor, and not a filename. Thus, we use 2>&1. Consider >& to be a redirect merger operator.