in reply to Re: Perl Script own path
in thread Perl Script own path

On some Unix-systems (Linux among them) you can actually assign to $0 to change the way the process shows up in the process list (I've seen that done e.g. to prevent passwords that are supplied on the commandline to show up with ps).

I now wonder if you changed your $0 to something else - would there be any way to recover the true script-path again?

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Re^3: Perl Script own path
by Marshall (Canon) on Jul 25, 2011 at 05:21 UTC
    I was quite astonished! I put an $0 = 'xyzzy"; statement in my Perl code and I'll be, ps -ef shows xyzzy in the process table! I am shocked and don't know why this could happen! I would have never thought that $0 was a writable variable. My brain was thinking along the lines of C and its char** argv, which is normally used as "read only", but that's not what is happening here.

    I guess you could save: my $saved = $0; or make a { local $0; $0 = 'xyzzy'; other stuff}, but I'm not sure why that would be useful?

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      My brain was thinking along the lines of C and its char** argv, which is normally used as "read only"

      argv[0] isn't read-only in C, either:

      /tmp>cat foo.c
#include <string.h>
#include <stdlib.h>
#include <unistd.h>

int main(int argc, char ** argv)
{
        pid_t pid;
        int status;

        strcpy(argv[0],"Did you expect this?");

        pid=fork();
        if (pid<0) {
                perror("Can't fork");
        }
        if (pid==0) {
                execlp("ps","ps","-f",NULL);
                exit(127);
        } else {
                waitpid(pid,&status,0);
        }
        exit(0);
}

/tmp>make foo
cc     foo.c   -o foo

/tmp>./foo
UID        PID  PPID  C STIME TTY          TIME CMD
foken     1255  1254  0 15:06 pts/0    00:00:00 -bash
foken     1676  1255  0 15:21 pts/0    00:00:00 Did you expect this?
foken     1677  1676  0 15:21 pts/0    00:00:00 ps -f

/tmp>

      [download]

      Of course, manipulating argv[] is not portable. Most Unixes support it in a more or less limited way. The example from above runs on linux 2.6.29.6, and exibits the typical Linux behavior. BSDs and other Unixes may show a different behaviour. On Windows, such a program would only manipulate a copy of the original command line, generated by the C runtime library. (Windows has no concepts like fork(), exec(), or argc/argv[]. Windows programs are expected to parse a single string containing all arguments.)

      Alexander

      --
      Today I will gladly share my knowledge and experience, for there are no sweeter words than "I told you so". ;-)
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[d/l]

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