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| Author |
Message |
herasleftnut
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Funky Unix code
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Oct 4 03:34 UTC 2006 |
The following is supposed to simulate
$ls;who
#include <sys/types.h>
#include <unistd.h>
#include <stdio.h>
int main(void) {
pid_t pid;
unsigned char *fname =";who";
if( (pid=fork()) < 0 ) {
fprintf(stderr, "fork error \n");
} else if (pid == 0) {
execl("/bin/ls","ls",fname,(char *)0);
}
exit(0);
}
However, when I run the code, I get
$gcc exc.c -o exc
$./exc
$ls: ;who: No such file or directory
The question is can I pass a string like ";pwho" without having to use
additional args on execl() ?
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| 10 responses total. |
cross
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response 1 of 10:
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Oct 4 04:41 UTC 2006 |
I think you're mixing ideas. "ls;who" is a something the shell does.
execl() is a library function that wraps a system call; it does no
interpretation of commands. So it has just passes ";who" as the 1st
argument to ls, which then takes over and tries to list that file (which
doesn't exist). I think what want is something more like the following:
/*
* Simulate some shell stuff.
*/
#include <stddef.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
static void
spawn(char *path, char *argv0)
{
pid_t pid;
pid = fork();
if (pid < 0) {
perror("fork");
exit(EXIT_FAILURE);
} else if (pid == 0) {
execl(path, argv0, NULL);
perror(path);
_exit(EXIT_FAILURE);
} else
wait(NULL);
}
int
main(void)
{
spawn("/bin/ls", "ls");
spawn("/usr/bin/who", "who");
return(EXIT_SUCCESS);
}
Note: I wrap up all the fork()/exec()/wait() stuff in a helper function.
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herasleftnut
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response 2 of 10:
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Oct 4 12:52 UTC 2006 |
Let's see if I'm understannding this correctly. My original code would be
equivalent to say like:
ls -who
vs something like ls -al.
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herasleftnut
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response 3 of 10:
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Oct 4 13:12 UTC 2006 |
Never mind, i just did
ls who
at the shell prompt.
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mcnally
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response 4 of 10:
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Oct 4 14:01 UTC 2006 |
Actually, it'd be more like:
ls ';who'
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cross
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response 5 of 10:
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Oct 4 19:08 UTC 2006 |
What Mike said. In fact, it's *exactly* like ls ';who' (note the quotes).
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herasleftnut
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response 6 of 10:
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Oct 5 00:47 UTC 2006 |
And I just found this wonderful function called setproctitle().
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herasleftnut
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response 7 of 10:
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Oct 5 01:31 UTC 2006 |
And you want to hear something really horrible? Going off topic, it took me
7 months to clue in on the Quicksort algorithm. I'm thinking I go too hung
up on the mathematics or funky coding.
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cross
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response 8 of 10:
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Oct 5 03:26 UTC 2006 |
Ack! Sorting is the type of thing that you shouldn't know, but that you
should've forgotten. (Well, I guess it's unfair to say that you shouldn't
know it - you should at least know it well enough to understand it.) But in
general, you're better off using a library to do your sorting for you instead
of rolling your own.
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herasleftnut
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response 9 of 10:
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Oct 6 00:29 UTC 2006 |
Okay, i was thinking more about the shell thingy at Labor Ready. Anyhow, how
when I go like
$sh -c "echo \"test\" "
test
it doesn't print
"test"
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cross
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response 10 of 10:
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Oct 6 17:56 UTC 2006 |
Because you're at the shell, and you're invoking the shell as a program to
do something for you. The shell that you're invoking (think of it was the
2nd instance of the shell, if you like) is interpreting the double quotes that
you've escaped. So, "echo \"test\" " is handled by the 1st shell (the one
you're interacting with) and passes the string ``echo "test" '' to the 2nd
shell. The second shell invokes ``echo'' with the string ``test'' (minus
quotes, which it interpreted) as the first argument.
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