Cory Petkovsek wrote:
> "eldump -? 2>&1 | more"
>
> I figure this means take standard error(2) and merge it with standard
> out(1). Right?
Right.
> Here are some experiments I ran:
> $ prog 1>d 1>j
>
> stdout went to j
> d is empty
But note that d was created. (-:
> $ prog 1>d 2>&1 1>j
>
> stdout went to j
> stderr went to d
>
> How many streams are there? Can I do something like
> $ prog 1>&4 4>&3 1>j 3>&5 5>&2
In the normal case, there are three:
0 = stdin
1 = stdout
2 = stderr
But a program can create more.
Let's say you had this script.
$ cat njturtles
> Can I fork a stream? Maybe like:
> $ prog 1>&3>&4 3>file1 4>file2
Use tee(1) to fork.
> What other tricks can I do with streams? Can I take stdout and route it in
> to stdin, thus making a superconducter inside my shell?
Heh, heh. I like the term, "superconductor".
I don't see a way to write a pipe loop using shell syntax. But it's
easy enough to do using C. Here's superconductor.c.
#include <stdio.h>
#include <sys/fcntl.h>
#include <unistd.h>
void child(int pipends[2], char *argv[], char *envp[])
{
char *child_argv[2];
if (dup2(pipends[O_RDONLY], 0) != 0) { /* stdin now comes from pipe */
perror("child dup2 stdin");
exit(1);
}
if (dup2(pipends[O_WRONLY], 1) != 1) { /* stdout now goes into pipe */
perror("child dup2 stdout");
exit(1);
}
close(pipends[0]);
close(pipends[1]);
child_argv[0] = "/bin/cat";
child_argv[1] = NULL;
execve(child_argv[0], child_argv, envp);
perror("child execve");
exit(1);
}
void parent(int pipends[2], char *argv[], char *envp[])
{
if (dup2(pipends[O_WRONLY], 1) != 1) { /* parent stdout goes into pipe */
perror("parent dup2 stdout");
exit(1);
}
close(pipends[0]);
close(pipends[1]);
argv[0] = "/bin/cat";
execve("/bin/cat", argv, envp);
perror("parent execve");
exit(1);
}
int main(int argc, char *argv[], char *envp[])
{
int pipends[2];
pid_t childpid;
if (pipe(pipends)) {
perror("pipe");
exit(1);
}
childpid = fork();
if (childpid < 0) {
perror("fork");
exit(1);
}
if (childpid == 0)
child(pipends, argv, envp);
else
parent(pipends, argv, envp);
return 0; /* never reached */
}
Run it under strace and give it a single line of input.
echo foo | strace -f ./superconductor
Hours of fun for the whole family!
--
Bob Miller K<bob>
kbobsoft software consulting
http://kbobsoft.com [EMAIL PROTECTED]
