You can also define your own file descriptors, and use them in
creative and useful ways within your scripts:

http://tldp.org/LDP/abs/html/io-redirection.html


On Thu, May 12, 2011 at 1:52 PM, Dale Farnsworth <dale@farnsworth.org> wrote:
>> Hi,
>>
>> I keep seeing a reference to what looks like two pre-populated variables
>> in shell scripts.  Actually this one is in a crontab.
>>
>> Something like this :  /home/user-name/directory-name/scriptname.php
>> >/dev/null 2>&1
>>
>> What is the number 1 and 2?  Is there others?
>
> Unix and copycats, like Linux use small non-negative integers to identify
> files that may be read from or written to.  They are known as file
> descriptors.  By convention, the first 3 file descriptors have a special
> meaning.  0 is standard input, 1 is standard output, and 2 is standard
> error.
>
> Normally, each process inherits these 3 file descriptors from its
> parent process but the shell allows you to "redirect" them to
> other files.
>
> Examples:
>        echo hello >newfile     # Creates the file "newfile" and writes
>                                # "hello\n" to it.
>        echo hello >/dev/null   # Writes "hello\n" to /dev/null, where it
>                                # disappears
>        echo hello 2>newfile2   # Creates the file "newfile2" and puts
>                                # error messages from "echo hello" into it.
>                                # Since "echo hello" won't output any error
>                                # messages, "newfile2" will be an empty file.
>        echo hello 2>&1         # Redirects the error output of the command
>                                # into the same file where the standard
>                                # output goes.
>
> So, "scriptname.php >/dev/null 2>&1" says to redirect the standard output
> of scriptname.php into the bit bucket and then to redirect the standard
> error output to the same place.
>
> -Dale
>
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