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Re: Array for system() call

by MZSanford (Curate)
on Oct 23, 2001 at 16:53 UTC ( #120770=note: print w/ replies, xml ) Need Help??


in reply to Array for system() call

When system() gets a scalar, it goes to the shell to execute the command. This means that you open a new shell instance (such a ksh), and run the program, including any redirection.

If system() gets an array, it instead does the fork() and then uses exec() to start the process. This by-passes the shell, so shell redirection does not work.


i had a memory leak once, and it ruined my favorite shirt.


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Re (2): Array for system() call
by dmmiller2k (Chaplain) on Oct 23, 2001 at 22:09 UTC

    To elaborate further, use this (note double quotes): system("@Call"); instead of this: system(@Call); to achieve the same effect as your first fragment.

    And, BTW, you probably meant to do this:

    $Call[0] = "gawk"; $Call[1] = "-f"; $Call[2] = "test.gawk"; $Call[3] = "test.file"; $Call[4] = ">"; $Call[5] = "out.file";

    or (slightly more efficiently) this:

    @Call = ( "gawk", "-f", "test.gawk", "test.file", ">", "out.file" );

    rather than your original code, which is assigning values to 1-element array slices, which are sublists -- NOT arrays! -- of references to the elements of the @Call array; needless to say, although it works, it is somewhat less efficient than either of the above approaches, and bad practice besides.

    dmm

    Give a man a fish and you feed him for a day ...
    Teach the man to fish and you feed him for a lifetime
    
      Here is an example of why using @arr[0] is just asking from trouble...
      #!/usr/bin/perl -wT use strict; my @arr; @arr[0] = localtime(); $arr[1] = localtime(); print "\$arr[0] = $arr[0]\n"; print "\$arr[1] = $arr[1]\n";
      Any guesses about the output of this simple script? Select area below to find out.

      =OUTPUT Scalar value @arr[0] better written as $arr[0] at ./arrayslice.pl line + 6. $arr[0] = 58 $arr[1] = Tue Oct 23 14:30:58 2001
      5 points for anyone who expected $arr[0] to contain an integer. 20 points for anyone who knew it would contain the seconds value, *not* the number of elements in the list returned by localtime().

      (10 points for anyone who coded up an example because they weren't sure... woohoo)

      -Blake

        Thanks for the example, blakem

        dmm

        You can give a man a fish and feed him for a day ...
        Or, you can teach him to fish and feed him for a lifetime
        

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