Beaker wrote,
>[...]
>Anyways, I've got things working but I'd really like to know what
>the "3" is all about. I read fd(4) which says 3-19 are "user-defined";
>so where is it being defined here ?
Right before the line that invokes gs:
> exec 3>&1 1>&2
The "3>&1" says, "Whenever anybody writes to descriptor 3, send it to
whatever descriptor 1 (standard output) was pointing to." The "1>&2"
says, "Whenever anybody writes to descriptor 1 (standard output), send it to
whatever descriptor 2 (standard error) was pointing to."
Thus when gs outputs to /dev/fd/3, it goes to the shell script's standard
output.
For the terminally curious, the redirection is implemented using the
dup2() system call. The above exec command executes these system calls:
dup2(1, 3);
dup2(2, 1);
Read "man dup2" to find out what that means.
- Neil Parker
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