File Descriptor redirection and duplication

Part I

It may help to think of the file descriptors as variables that contain a value, and the value determines where the output goes (STDOUT, STDERR, a file, etc).

When redirecting 2 (STDERR) to 1 (STDOUT), the following happens for the the two cases you described:

1) Redirect STDERR before STDOUT

[root@testsrv3 ~]# ls -alt FileNotThere File 2>&1 > logfile
ls: cannot access FileNotThere: No such file or directory
[root@testsrv3 ~]# cat logfile
-rw-r--r-- 1 root root 0 2010-02-26 05:34 File

Here we are changing FD 2 to point to the same target FD 1 CURRENTLY points at, STDOUT. Then we change FD 1 to redirect to a file, logfile. This leaves FD 2 still containing the target location of STDOUT, since it now contains the value of FD 1 at the time it was redirected.

2) Redirect STDERR after STDOUT

root@testsrv3 ~]# ls -alt FileNotThere File > logfile 2>&1
[root@testsrv3 ~]# cat logfile
ls: cannot access FileNotThere: No such file or directory
-rw-r--r-- 1 root root 0 2010-02-26 05:34 File

Here we are redirecting FD 1 to a file, logfile, and then redirecting FD 2 to the same location as FD 1 is pointing, which is currently logfile. Thus, both the default file desciptors for STDIN and STDOUT sending inputs to the file.

So, as you can see, it can be useful to think of file descriptors as variables holding a value, and by redirecting them you aren't chaining them through another FD, but reassigning their current value to the same value as the target FD.

Indeed, reading the bash man page shows they refer to the 2>&1 syntax as a file descriptor duplication, not redirection, which is the case when used without & (redirect inputs to a file).

Part II

I have to admit I'm a bit unsure about how this is functioning, but I assume that FD 5 is opened for input, and the default action for output is to display on STDOUT, which is why you can echo a string to FD 5 afterwards and see it on STDOUT. That's a complete guess, but it makes sense to me that the input and output portions of a FD would take STDIN and STDOUT as their default values.