Tech9 min
What chmod 755 Actually Means (and Why 777 Isn't the Fix)
755 is not a number. It is nine switches written in shorthand, three for the owner, three for the group, three for everyone else. Learn the 4-2-1 trick once and you stop looking it up. Then learn the part almost nobody knows: most permission denied errors on a file are really about a directory.
linuxchmodfile-permissions