3. All about redirection

3.1 Theory and quick reference

There are 3 file descriptors, stdin, stdout and stderr (std=standard).

Basically you can:

1. redirect stdout to a file
2. redirect stderr to a file
3. redirect stdout to a stderr
4. redirect stderr to a stdout
5. redirect stderr and stdout to a file
6. redirect stderr and stdout to stdout
7. redirect stderr and stdout to 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.

3.2 Sample: stdout 2 file

This will cause the ouput of a program to be written to a file.

        ls -l > ls-l.txt
        

3.3 Sample: stderr 2 file

This will cause the stderr ouput of a program to be written to a file.

        grep da * 2> grep-errors.txt
        

3.4 Sample: stdout 2 stderr

This will cause the stderr ouput of a program to be written to the same filedescriptor than stdout.

        grep da * 1>&2 
        

3.5 Sample: stderr 2 stdout

This will cause the stderr ouput of a program to be written to the same filedescriptor than stdout.

        grep * 2>&1
        

3.6 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.

        rm -f $(find / -name core) &> /dev/null