Unix UID Size – How Many Bits?

linuxuidunix

I understand that unix user IDs (UIDs) are usually 16 or 32 bit unsigned integers but how can I find out for any given system (in a shell)?

Best Answer

You'll need to look in <limits.h> (or one of the files it includes, e.g., sys/syslimits.h on OS X) for the #define of UID_MAX.

Most recent operating systems (Solaris 2.x, OS X, BSD, Linux, HP-UX 11i, AIX 6) can handle up to two billion (2^31-2), so I would assume that and make a workaround for the more obscure systems that don't.

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Linux – Multiple *NIX Accounts with Identical UID

My opinion:

Is this ability designed or is it just the way it happens to work?

It is designed. Since I started using *NIX, you have been able to place users on common groups. The ability to have the UID be the same without problems is just an intended result that, like everything, might bring problems if incorrectly managed.

Is this going to be consistent across *nix variants?

I believe so.

Is this accepted practice?

Accepted as in generally used in one way or another, yes.

Are there unintended consequences to this practice?

Other than login auditing, you have nothing else. Unless you wanted exactly that, to start with.

Directories on Unix and Unix-like Systems – Understanding Their Meaning

For more data on the layout of Linux file-systems, look at the Filesystem Hierarchy Standard (now at version 2.3, with the beta 3.0 version deployed on most recent distros). It does explain some of where the names came from:

/bin - Binaries.
/boot - Files required for booting.
/dev - Device files.
/etc - Et cetera. The name is inherited from the earliest Unixes, which is when it became the spot to put config-files.
/home - Where home directories are kept.
/lib - Where code libraries are kept.
/media - A more modern directory, but where removable media gets mounted.
/mnt - Where temporary file-systems are mounted.
/opt - Where optional add-on software is installed. This is discrete from /usr/local/ for reasons I'll get to later.
/run - Where runtime variable data is kept.
/sbin - Where super-binaries are stored. These usually only work with root.
/srv - Stands for "serve". This directory is intended for static files that are served out. /srv/http would be for static websites, /srv/ftp for an FTP server.
/tmp - Where temporary files may be stored.
/usr - Another directory inherited from the Unixes of old, it stands for "UNIX System Resources". It does not stand for "user" (see the Debian Wiki). This directory should be sharable between hosts, and can be NFS mounted to multiple hosts safely. It can be mounted read-only safely.
/var - Another directory inherited from the Unixes of old, it stands for "variable". This is where system data that varies may be stored. Such things as spool and cache directories may be located here. If a program needs to write to the local file-system and isn't serving that data to someone directly, it'll go here.

/opt vs /usr/local

The rule of thumb I've seen is best described as:

Use /usr/local for things that would normally go into /usr, or are overriding things that are already in /usr. Use /opt for things that install all in one directory, or are otherwise special.

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Related Solutions

Linux – Multiple *NIX Accounts with Identical UID

Directories on Unix and Unix-like Systems – Understanding Their Meaning

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