Theory and quick reference
There are 3 file descriptors, stdin, stdout and stderr (std=standard).
Basically you can:
redirect stdout to a file
redirect stderr to a file
redirect stdout to a stderr
redirect stderr to a stdout
redirect stderr and stdout to a file
redirect stderr and stdout to stdout
redirect stderr and stdout to stderr
1 ‘represents’ stdout and 2 stderr.
A little note for seeing this things: with the less command you can view both stdout (which will remain on the buffer) and the stderr that will be printed on the screen, but erased as you try to ‘browse’ the buffer.
Sample: stdout 2 file
This will cause the ouput of a program to be written to a file.
Here, a file called ‘ls-l.txt’ will be created and it will contain what you would see on the screen if you type the command ‘ls -l’ and execute it.
Sample: stderr 2 file
This will cause the stderr ouput of a program to be written to a file.
Here, a file called ‘grep-errors.txt’ will be created and it will contain what you would see the stderr portion of the output of the ‘grep da *’ command.
Sample: stdout 2 stderr
This will cause the stderr ouput of a program to be written to the same filedescriptor than stdout.
Here, the stdout portion of the command is sent to stderr, you may notice that in differen ways.
Sample: stderr 2 stdout
This will cause the stderr ouput of a program to be written to the same filedescriptor than stdout.
Here, the stderr portion of the command is sent to stdout, if you pipe to less, you’ll see that lines that normally ‘dissapear’ (as they are written to stderr) are being kept now (because they’re on stdout).
Sample: stderr and stdout 2 file
This will place every output of a program to a file. This is suitable sometimes for cron entries, if you want a command to pass in absolute silence.
This (thinking on the cron entry) will delete every file called ‘core’ in any directory. Notice that you should be pretty sure of what a command is doing if you are going to wipe it’s output.