How to use Erlang file:read_file_info permissions/mode info?

Question

The Erlang docs for file:read_file_info/1 state "file permissions [are] the sum" and "other bits...may be set", not instilling confidence. And, Google has not been my friend here.

I'm looking to take the mode returned by file:read_file_info/1 , eg 33188 , on a Linux machine and convert it into something more human readable and/or recognizable, like rw-r--r-- or 644 .

Any tips, links, or directions greatly appreciated.

Answer 1

The short way:

io_lib:format("~.8B", [Mode]).

... or:

io_lib:format("~.8B", [Mode band 8#777]).

For Mode = 33204 these two will give you respectively: ["100664"] and ["664"] .

The long way:

print(Mode) ->
    print(Mode band 8#777, []).

print(0, Acc) when length(Acc) =:= 9 ->
    Acc;
print(N, Acc) ->
    Char = perm(N band 1, length(Acc) rem 3),
    print(N bsr 1, [Char | Acc]).

perm(0, _) ->
    $-;
perm(1, 0) ->
    $x;
perm(1, 1) ->
    $w;
perm(1, 2) ->
    $r.

This one (function print/1 ) for Mode = 33204 will give you this as result: "rw-rw-r--" .

---

If something was unclear for one, I'll try to expound basic things behind the snippets which I have provided.

As @macintux mentioned already, the 33204 in fact is a decimal representation of the octal number 100664. These three lowest octal digits ( 664 ) there is probably what you need, and so we get them with bitwise and ( band ) operation with the highest number which fits in three octal digits ( 8#777 ). That's why short way is so short - you just tell erlang to convert Mode to string as if it was the octal number.

The second representation you've mentioned (like rw-rw-r-- , something that ls spits out) is easily reproducible from binary representation of the Mode number. Note that three octal digits will give you exactly nine binary digits ( 8#644 = 2#110110100 ). In fact this is the string rwxrwxrwx where each element replaced by - if corresponding digit equals 0 . If digit is 1 the element remains untouched.

So there is slightly cleaner approach to achieve this:

print(Mode) ->
    print(Mode band 8#777, lists:reverse("rwxrwxrwx"), []).

print(0, [], Acc) ->
    Acc;
print(N, [Char0 | Rest], Acc) ->
    Char = char(N band 1, Char0),
    print(N bsr 1, Rest, [Char | Acc]).

char(0, _) ->
    $-;
char(1, C) ->
    C.

---

I hope you got the point. Anyway feel free to ask any questions in comments if you doubt.