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Why is /tmp a symlink to /private/tmp on Mac OS X? In other words, why isn't /tmp just a regular directory, like on Linux or BSD? I understand how it works and I don't mind it, I'm just interested in the (historical?) reasoning behind it.
As I understand it, it's a holdover from NextStep (which OS X is based on), and NextStep did it to support NetBooting. The idea was that you could boot from a network-hosted volume (probably read-only, and certainly shared with other computers), and early in the boot process mount a local (writable) volume on /private; as g mentioned, this allowed runtime-modification of /var and /tmp, as well as per-computer settings in /etc.
This isn't needed anymore, as Apple's current NetBoot system uses a shadow disk image to store changes anywhere on the boot volume. But some programs/docs/etc now assume the files live under /private, so it'd be too much trouble to switch them back...
Update: since I wrote this, Apple has stopped supporting NetBoot, so the original purpose of /private is even more obsolete. However, in macOS Catalina (version 10.15), they've added a new volume split. In this case it's for security rather than to support NetBoot, but it works in a fairly similar way.
Catalina's system volume is mounted read-only, with a read-write volume mounted at /System/Library/Data (analogous to the old system that mounted a RW volume at /private), and "firmlinks" making parts of the RW volume appear at their usual locations in the filesystem (again, analogous to the symbolic links that make parts of /private appear at their usual locations). For example, /Users is now a firmlink to /System/Library/Data/Users. The Eclectic Light Company has a good summary .
Catalina also still has the symbolic links to /private; thus, when you access /etc on Catalina, it follows the symlink to /private/etc, and then the firmlink to /System/Library/Data/private/etc
I have always wondered the same thing. I can't find any documentation to support it, but typically this pattern is used to make it easier to store files on another volume (e.g. hard drive). This allows the drive to be mounted (e.g. attached) into the file system in one place. For example when the drive is mounted at /private and then the folders /etc , /tmp , and /var are then all located on that other drive.
What I can't say is why this would be beneficial. It is worth noting, however, that these three folders contain "data" files such as configuration, temporary, log, transient and database files rather than executable code contained in the /bin , /sbin and /usr folders.
Not sure about the historical reason, but OS X always “reorganized” the typical Unix structure. /tmp is not the only thing that goes to /private , it also has /etc and /var .
Maybe someone with more OS X background can come up with something more reasonable.
/tmp is a symbolic lynk to /private/etc so as to maintain clearly separated 2 filesystems:
/ which can be mounted as read-only, to protect it against
any accidental or unwanted modification, and to protect it of being filled with always increasing files (logs and temporary files),
/private which can be mounted as read-write, and which hold any directory containing modifiable files.
If you look at / you will notice 3 directories which are similar
symbolic links for this same reason:
This separation of access between read-write and read-only filesystems
isn't used actually (in MacOS X), but everything is in place to achieve
this security separation.
Some admins are enforcing this security separation by defining a specific /private
filesystem with the appropriate size and appropriate mounting options
(most notably nosuid ).
Frequently in Unix, identical ways of the same thing are because of historical differences between System V Unix and BSD Unix. Modern Unixes have to support both to be compatible.
For instance, lpr and lp for printing: lpr is from BSD and lp is from System V.
Whether this is the case here, I don't know.
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Temporary files (also called "temp files") usually have filenames that end with the extension .TMP . Though they share the same file extension, temporary files can be created by many different programs. This can make it difficult to figure out what program created a specific temp file, or even where the file came from.
Sometimes, the part of the filename before the extension will give you a clue as to where the file is from. For example, if the filename is 2006_annual_report.tmp , the file might be a Word document or Excel spreadsheet that was named "2006 annual report." Other times, the location of a temp file may help you determine what program created the file. For example, if you find a temp file in a folder named "Temporary Files" within the main folder of a certain application , the temporary file most likely belongs to that application.
Programs create temporary files primarily for backup purposes. Many programs, such as those included with Microsoft Office, will save a temporary version of a file every few minutes while the original file is open. Then if you have not saved the file and the program unexpectedly crashes or the computer shuts down, there will be a temporary file that was recently saved. Temp files are typically deleted automatically when the program is closed normally, but if the program quits unexpectedly, the temp files are not deleted.
However, some temp files are left behind even when a program is quit correctly. This may be due to programming errors that cause temporary files to not be correctly disposed of when an application is quit. These files can accumulate on your hard drive without you even noticing.
Finally, Web browsers , such as Internet Explorer, create "Temporary Internet Files" to store user data, including cookies , images from visited Web pages, and the user's browsing history.
If you plan on manually removing specific temp files, first make sure the files you want to delete are actually temporary files and that you do not need them for any reason. Once you have verified that they are not needed or are duplicates of other files, simply move the files to the Recycle Bin (Windows) or the Trash (Mac) and empty the trash.
If you want to delete many temp files at once, several programs are available that will automate the process for you. These programs can search your hard disk for temp files and remove the ones you don't need, saving hard disk space. Some examples of Windows utilities include:
NOTE: If you want to delete Temporary Internet Files in Windows, follow these instructions .
Open common file formats with a file viewer for your platform.

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Is it safe to delete tmp files in the Hidden /private/tmp file?
On a similar note:
Can I delete files from the /private/var/tmp directory?

There are over 100 files in there, some going back to 2007, taking up over 5GB.

Thanks
Dave

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why do you want to delete anything from there by hand? programs put some temporary items in /private/tmp and they might need them while they are running. but there is no need to clean up this folder by hand. programs should remove any temp files from that directory when you quit them and if not, it should be emptied by the system when you reboot. when I look at my /private/tmp I see a couple of folders which seem to belong to launchd which is running at all times. I wouldn't touch those. what is it you want to delete?

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I've been using 1Password 3 beta and everytime I install a new version it writes the app to it's own folder in the tmp file. I have deleted it in the past and it had no ill effects on the app running normally.

Call it curiosity.

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if you quit 1password, you can safely delete any 1password folders left there.

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On a similar note:
Can I delete files from the /private/var/tmp directory?

There are over 100 files in there, some going back to 2007, taking up over 5GB.

Thanks
Dave

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