On my mac I'm using an unjournaled HFS partition to share files between OSX 10.8 and Ubuntu 12.04.
It was a nice thought at first, because Time Machine will automatically backup the volume in OSX, but I soon noticed that OSX and Ubuntu mess with the permission in a way that makes things messy for me.
So, in order to fully view and change files, I keep using chmod
to apply permissions that will allow me to fully use a document. But I don't understand why I have to keep applying changes over and over.
Is possible to set some kind of permission permanently so that both operating systems will respect permanently?
I guess 777 will work, but I thought that this is not a smart thing to do. But as long as 'others' does not get full access (third seven), I see a lock icon on the file in ubuntu.
Best Answer
User names are irrelevant. Permissions in both HFS+ and Linux-native filesystems are stored in terms of user IDs (UIDs), which are numbers associated with usernames. In Ubuntu, as in most modern Linux distributions, the first user is given a UID of 1000 by default. In OS X, the first user is given a UID of 501 by default. Thus, when sharing media that encode UID values, the UID values are likely to not match.
One way to fix this is by setting loose permissions (the mode value, as in
rwxr-xr-x
, or 755 in octal). Note that the permissions octal code is not the same as the UID value. In either OS, you can set the default permissions used on files with theumask
command, which specifies the bit value to be removed from file permissions. For instance,umask 022
removes write permission for the group and other permissions, resulting in 755 (rwxr-xr-x
) permissions on new files (or 644 if something removes the execute permission bit, which is common practice for files). This is largely a command-line tool, though; if you're largely a GUI user, you'll need to find another tool to do the job, probably related to your desktop environment's defaults. This may be obscure and poorly documented. Also, setting loose permissions in this way can have security drawbacks, especially if yours is a multi-user system.A better approach is to synchronize your account UIDs across Linux and OS X. You can easily change the UID value in Linux with the
usermod
command, as in:This command sets the UID for
dale
to 501. There are some significant caveats, though:usermod
must be used asroot
. You can execute it viasudo
, but doing so from the account you're modifying is inadvisable in the extreme. Thus, you'll need to either giveroot
a password and log intoroot
directly or usesudo
from a second user account.usermod
command won't change the ownership of any files owned by the user in question. To adjust ownership of those files, you'll need to locate them and then change their ownership withchown
. Most of the files will be in the user's home directory, sochown -R dale: /home/dale
, typed asroot
after changingdale
's UID, will change most ofdale
's files to use the new UID number. Some of the user's files may be located elsewhere, though. Typingfind / -uid 1000
will find all the files that use the old UID (assuming it was 1000). Note that thisfind
command will probably take several minutes to complete. To speed it up, unmount any filesystems on which you're sure it will find no hits, such as FAT or NTFS volumes./etc/fstab
entry, though, you may need to adjust the UID value it specifies./etc/login.defs
. If you fail to change theUID_MIN
value in this file, you'll likely discover that your account will seem to "disappear" from the GUI login screen, and perhaps from some other systems. Thus, you should edit that file.In theory, you could change the UID of your OS X account(s) in a similar way to achieve the same goal. I'm less familiar with the OS X account-maintenance tools, though, so I can't provide explicit instructions for doing so. Adjusting the OS X values would have the advantage of your not having to adjust
UID_MIN
in Linux.If you've got multiple accounts on your computer, you should adjust them all to keep them all synchronized across your OS installations.
One more point: The Group ID (GID) value is stored in a similar way. IIRC, Ubuntu assigns a GID value for each account that's identical to its UID value. I don't recall what OS X does by default. You might want to adjust the GID values for the two OSes in a way that's analogous to the UID changes, but this isn't likely to be as important as adjusting the UID values.
EDIT: If you want to change your UID (and GID, if desired) in macOS/OS X rather than in Ubuntu, you can do so. As this modification in macOS is beyond the scope of this site, I'll just link to a few pages that provide procedures for doing this in macOS: