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I read this up on this website and it doesn't make sense.

http://rcsg-gsir.imsb-dsgi.nrc-cnrc.gc.ca/documents/basic/node32.html

When UNIX was first written, /bin and /usr/bin physically resided on two different disks: /bin being on a smaller faster (more expensive) disk, and /usr/bin on a bigger slower disk. Now, /bin is a symbolic link to /usr/bin: they are essentially the same directory.

But when you ls the /bin folder, it has far less content than the /usr/bin folder (at least on my running system).

So can someone please explain the difference?

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  • aren't those guys talking about a specific machine?
    – tshepang
    Jan 15, 2011 at 11:57
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    @tshepang well if you keep clicking up, in the article you'll reach Basic Introduction to UNIX/linux. So I'd say no, they're not talking about a specific machine
    – xenoterracide
    Jan 15, 2011 at 12:08
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    @xeno Yeah, I did, but didn't stick around long enough. In that case, that page must be taken down. So misleading.
    – tshepang
    Jan 15, 2011 at 12:10
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    That document can be considered 'historical reference'. If you look at the "about this document" page, you will see that it was copyrighted originally in 1993. If you look at the bibliography, it's most recent source is 1997. Most of the popular unix-like operating systems have changed substantially since then.
    – gabe.
    Jan 15, 2011 at 15:02
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    Anyone interested in the historicity of this configuration should read this: lists.busybox.net/pipermail/busybox/2010-December/074114.html
    – JDS
    Nov 30, 2016 at 19:02

5 Answers 5

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109

What? no /bin/ is not a symlink to /usr/bin on any FHS compliant system. Note that there are still popular Unices and Linuxes that ignore this - for example, /bin and /sbin are symlinked to /usr/bin on Arch Linux (the reasoning being that you don't need /bin for rescue/single-user-mode, since you'd just boot a live CD).

/bin

contains commands that may be used by both the system administrator and by users, but which are required when no other filesystems are mounted (e.g. in single user mode). It may also contain commands which are used indirectly by scripts

/usr/bin/

This is the primary directory of executable commands on the system.

essentially, /bin contains executables which are required by the system for emergency repairs, booting, and single user mode. /usr/bin contains any binaries that aren't required.

I will note, that they can be on separate disks/partitions, /bin must be on the same disk as /. /usr/bin can be on another disk - although note that this configuration has been kind of broken for a while (this is why e.g. systemd warns about this configuration on boot).

For full correctness, some unices may ignore FHS, as I believe it is only a Linux Standard, I'm not aware that it has yet been included in SUS, Posix or any other UNIX standard, though it should be IMHO. It is a part of the LSB standard though.

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  • So does it mean that I can share /usr/bin between two distros? like ubuntu 10.04 and 10.10?
    – balki
    Jan 15, 2011 at 12:48
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    @balki no, they still have to be linked correctly, which means they'd have to be using the right version of the libs they are built against.
    – xenoterracide
    Jan 15, 2011 at 13:07
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    /bin is a link to /usr/bin on some unices, such as (some releases of?) Solaris.
    – Gilles 'SO- stop being evil'
    Jan 15, 2011 at 16:08
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    @balki you might be able to share /usr between two very similar distro's, though I wouldn't try it unless you know what you are doing, and you are prepared for the experiment to fail in a way that makes the computer not boot, or eat your data. Also be advised that many of the big differences between 10.04 and 10.10 will be in /usr
    – xenoterracide
    Jan 20, 2011 at 13:23
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    Let's also include that /usr/local/bin is where programs that are not included with your distribution but are intended for system-wide use are installed.
    – Donato
    May 2, 2015 at 6:17
90

Update: For some history and the real reason why there is /usr, read this: http://lists.busybox.net/pipermail/busybox/2010-December/074114.html. Also this.

/sbin - Binaries needed for booting, low-level system repair, or maintenance (run level 1 or S)

/bin - Binaries needed for normal/standard system functioning at any run level.

/usr/bin - Application/distribution binaries meant to be accessed by locally logged in users

/usr/sbin - Application/distribution binaries that support or configure stuff in /sbin.

/usr/share/bin - Application/distribution binaries or scripts meant to be accesed via the web, i.e. Apache web applications

*local* - Binaries not part of a distribution; locally compiled or manually installed. There's usually never a /local/bin but always a /usr/local/bin and /usr/local/share/bin.

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  • 6
    Oh, and you can put all these on a separate disk or partition if you like, for security or whatever. The idea is that you can umount /usr and the system will have all it needs for basic functioning, but it can't run any non-system applications.
    – LawrenceC
    Jan 19, 2011 at 19:16
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    I wish I would've known this years ago. Best explanation I have seen so far. I can't say I looked very hard, but glad to finally know the answer.
    – David
    Sep 27, 2013 at 7:20
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    this is a great answer. i'd donate you 10 of my own points if i could
    – amphibient
    Oct 24, 2013 at 15:16
  • @ultrasawblade is /usr/bin automatically unmounted when there are no users logged in? For example, when LINUX boots and presents the login authentication, is /usr/bin mounted or no?
    – user98877
    Jan 13, 2015 at 14:21
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    No, it's mounted at boot and stays mounted unless you unmount it, unless your system is configured unusually.
    – LawrenceC
    Jan 13, 2015 at 16:12
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Some kind of "update" on this issue:

Recently some Linux distributions are merging /bin into /usr/bin and relatedly /lib into /usr/lib. Sometimes also (/usr)/sbin to /usr/bin (Arch Linux). So /usr is expected to be available at the same time as /.

The distinction between the two hierarchies is taken to be unnecessary complexity now. The idea was once having only /bin available at boot, but having an initial ramdisk makes this obsolete.

I know of Fedora Linux (2011) and Arch Linux (2012) going this way and Solaris is doing this for a long time (> 15 years).

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8

On Linux /bin and /usr/bin are still separate because it is common to have /usr on a separate partition (although this configuration breaks in subtle ways, sometimes). In /bin is all the commands that you will need if you only have / mounted.

On Solaris and Arch Linux (and probably others) /bin is a symlink to /usr/bin. Arch also has /sbin and /usr/sbin symlinked to /usr/bin.

Of particular note, the statement that /bin is for "system administrator" commands and /usr/bin is for user commands is not true (unless you think that bash and ls are for admins only, in which case you have a lot to learn). Administrator commands are in /sbin and /usr/sbin.

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  • 1
    who said bin is only for system administrators? that's what sbin is for.
    – xenoterracide
    Jan 17, 2011 at 13:08
  • Misquoted you. Sorry, I was reading too fast.
    – bahamat
    Jan 17, 2011 at 16:23
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There are many UNIX-based systems. Linux, AIX, Solaris, BSD, etc. The original quote gives historical context that applies to all flavors. If you look on any one specific system, you will see different results. The last sentence of the original quote is specific to only some versions and distributions.

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