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Re^2: What does ">>" do? (And other stat questions)

by three18ti (Scribe)
on Nov 01, 2013 at 01:31 UTC ( #1060677=note: print w/replies, xml ) Need Help??

in reply to Re: What does ">>" do? (And other stat questions)
in thread What does ">>" do? (And other stat questions)

Hello Rolf

> Sorry, you're asking too many questions at the same time to be easily answered in one thread.

I think I agree with you. When I started typing, I really only wanted to know what ">>" was. But as I was typing it became clear to me that I don't understand more than I initially though (or I understand less than I initially thought).

Apparently I'm lacking some fundamental knowledge which is hampering my understanding of stat on the whole.

> First step, plz try to read the docs and tell us what remains unclear: perldoc perlop

Thanks for this, there's usually a perldoc that I've overlooked

> (BTW: perlop is the second hit for perl binary operator )

Thanks for this. My search for "perl binary" was clearly insufficient. And the first hit 1 sort of explains bitwise operators. I think the examples are doing EXACTLY the same thing (albeit with values for text formatting and not file permissions, but I think the concept is the same). But here's where I'm stumped. How does 128 become 10000000 amd 16 become 00010000? Is that just the integer converted to a binary number? The "or" makes sense in this case. I think if we & 10000000 and 00010000 we would get 0110111, right? How would this apply to octal numbers?

> '&' is not a part of the word but a "binary and" operator. But I agree that the missing space doesn't improve readability.

Ok, cool. So we're doing some binary operation on an integer and an octal "07777"? right? I thought binary operations had to be performed on integers? (And why would binary operations on integers be performed as floating point integers? That's more curiosity and I don't think important to my comprehension of the topic)

I think I kinda get WHAT a bitwise shift is, e.g. "00000100 << 2" would be 00010000

In the context of my program, or any program, what does that signify and when/why would I want to do it?

> Do you know how file-permissions are managed in unix-like systems?

After this thread, and speaking as an experienced Unix/Linux SysAdmin: apparently not...

In a nutshell, the permissions are represented as one digit in a three digit octal number. E.g. 0555 (chmod assumes octal so the 0 is often omitted) would be read and execute permissions for user, group, and other. likewise 0777 is the TRUE mark of the beast as it gives read, write, and execute permissions to everyone (and often "nobody" or "www-data"...)

Ok... so. if 4 is represented in binary as 100, 2 is 010, and 1 is 001, and you "or" 4 and 2 together:

100 | 010 = 110

Which is 6

HOLY CRAP I think it makes sense all of a sudden... And that answers my question about octal numbers, just like regular integers, they're just converted by decimal place? (well... kinda 0111 would be 001001001 right?)

So WTF is the deal with with "&07777"

if we're "and"ing I know that and returns the values that are the same... how does this help me? since 07777 is 111111111111 right? What practical implications does this have?

Literal octal numbers start with a 0 followed by octal digits. see perlnumber

Oh. An octal number IS an integer. Ok, that answers my question about bitwise operators.

> see also oct for a discussion of your code

That's a little bit cleaner. Is there a good way to compare octal numbers? (googling "perl compare octal values" returned your link and some really old links. none of which say you should compare them as strings (with "eq") or as integers. Some experimenting seems to indicate that perl can compare them as numbers, since they are...

perl -e 'print "yes\n" if "0777" == "0777"' perl -e 'print "yes\n" if "0777" == "0776"'

Thanks for your help. Even just your responses gave me a target in my research and responding to you helped my understanding greatly. Thanks so much for your help!


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Re^3: What does ">>" do? (And other stat questions)
by LanX (Bishop) on Nov 01, 2013 at 02:59 UTC
    again I don't know which of the questions are to be answered.

    All numbers here are integers, they differ only in literal representation - i.e. binary, octal, hex or decimal.

    Internally they are always only the same bits, i.e. you can mix representations in binary operations.

    07777 stands for 4 groups with 3 bits (2**3 =8 => octal).

    the highest group is quite special and include "exotic" things like sticky-bit. The others are for user, group and others with rwx -bits,

    But there are more bits set in mode to code the file-type:

    from stat

    Because the mode contains both the file type and its permissions, you should mask off the file type portion +and (s)printf using a "%o" if you want to see the real perm +issions. $mode = (stat($filename))[2]; printf "Permissions are %04o\n", $mode & 07777;

    Any questions left? =)

    Cheers Rolf

    ( addicted to the Perl Programming Language)

      Hey Rolf

      Thanks for all of your help. I know a couple of times I went off on a tangent there, but I would ask a question and then, I don't know if it's just typing out questions, but it gets my mind working on the question, and as you can see, I end up answering many of my own questions. I can see that that can be frustrating for someone like you to help me, but I really appreciate your help, especially getting my brain thinking about what I need to.

      I really appreciate all your help, I think I'm starting to get stat AND I learned a bunch about binary math and how octal flags work.

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