Question

I want to check whether a file on Linux has permission 555, but the file has a 755 permission instead.
So I do a boolean AND in my program:

if ((perm_of_file && perm_required) == 555)

i.e. (755 && 555 == 555)

Should this evaluate to true?
I know this is not intuitive from a programming prospective and the real background question is:
Is 755 considered as a “member” of the 555 permission group?

What I tried:
I use find -perm 555 on Linux and it doesn’t list those files who has 755 permission, so I am guessing the answer to the question is NO?

If somebody can help me answer or rephrase this question, that would be very helpful.
Thanks in advance!

Answer 1

• ((file_mode && some_bits) == some_bits) # is logical and, you need bitwise and
• ((file_mode & some_bits) == some_bits) # is bitwise and
• file modes are (still) expressed in octal notation, in most C-like languages you’ll need to add a leading zero to indicate octal mode
• command-line programs (such as chmod or find) don’t need the leading zero, they expect the numerical mode-string to be octal
• find . -mode xxx # finds files with exactly the mode xxx
• find . -mode -xxx # finds files with at least the mode bits xxx set