On Mon, Nov 3, 2008 at 06:19, Chet Ramey <[EMAIL PROTECTED]> wrote: > Clark J. Wang wrote: > > > When I was doing some testing I found the file descriptor 10 is always > > duplicate of fd 0 and it cannot be closed. > > Half right. When a redirection involving fd 0 is evaluated, the shell > has to save fd 0 somewhere so it can be restored. It uses fcntl to > duplicate fd 0 to something above the traditional user-addressable fd > range (0-9). (You can check the code in redir.c:add_undo_redirect(), > which happens to use fds 10 and above). > Thanks. I found the macro definition in redir.c:
#define SHELL_FD_BASE 10 > > Since there's no real guaranteed-safe file descriptor range for the > shell's use, it's possible, as you did, to use the fd bash uses to save > fd 0. And since fd 0 is saved before the rest of the redirection is > evaluated, state can change in the middle of the redirection's logical > evaluation. > > This doesn't really have anything to do with closing fd 10, which > succeeded. > > There are things that the shell can do to try and minimize such collisions, > and bash-4.0 will do a better job at it. > This is the first time of my hearing of bash-4.0. Looking forward to it. :-) > > Chet > > -- > ``The lyf so short, the craft so long to lerne.'' - Chaucer > > Chet Ramey, ITS, CWRU [EMAIL PROTECTED] > http://cnswww.cns.cwru.edu/~chet/ <http://cnswww.cns.cwru.edu/%7Echet/> >
