On Mon, Nov 22, 2010 at 8:37 AM, rajeevrkv wrote: > Aah, the subtleties of shell plumbing....
The way that plumbing works is fabulous. (I had read about this to an extent, but there were more details as you've explained below) > $ cat < foo > bash: foo: No such file or directory Yes, I failed to notice the difference initially, though I knew that '<' will cause an input to be redirected from the file to STDIN of process. (cat) > $ cat < foo 2> error > bash: foo: No such file or directory > > Remember, bash has no idea when the process may be expecting the input, it > just has to keep it available on the STDIN fd for whenever the process asks > to read it. In order to do that, bash FIRST creates the fd pointing to the > file, then creates the fd redirecting STDERR to another file and then > finally starts the process with the redirection in place. Now, if the > initial creation of the fd does not take place, "bash" thrown the error on > it's STDERR fd, not the STDERR fd of "cat"(Actually, the cat process is not > actually even started, since the redirection setup failed.) That's why the > redirection of the "cat" process STDERR does not capture the errror message. Thanks for the detailed explanation I did understand what you said. (Bash expands the meta-characters first and then processes the command). So effectively, whenever, we are redirecting to STDOUT or STDERR, the command (process) that is executed, its output or error is put into a file. Anytime, the input for bash fails (via file through < operator), the subsequent STDOUT and STDERR are reverted to usual (the terminal) [even if appeared to be changed in the actual command]. > I hope that this makes shell redirection a > little clearer for you. It does. It gives me more insight into the way Bash works. If there's a book / link, which provides such details with examples, I'll be able to understand bash better than I currently do. Thankyou very much. You've been a patient guy in explaining all this. (I remember / know you when I met you at one of the colleges explaining students about Fedora Linux) :) -- Roshan Baladhanvi -- http://mm.glug-bom.org/mailman/listinfo/linuxers
