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While trying to test something unconnected, I launched an infinite loop on my terminator terminal emulator which printed a lot of data to STDOUT. After running it for a few minutes, I noticed that my /tmp partition was full.

$ df -h
Filesystem                                   Size  Used Avail  Use%  Mounted on
rootfs                                        68G   21G   44G   33%  /
udev                                          10M     0   10M    0%  /dev
tmpfs                                         800M  1.2M  799M   1%  /run
/dev/disk/by-uuid/e29c6006c5c6                68G   21G   44G   33%  /
tmpfs                                         5.0M     0  5.0M   0%  /run/lock
tmpfs                                         3.2G  140K  3.2G   1%  /run/shm
/dev/sda6                                     290G  204G   72G  75%  /home
/dev/sda3                                      79G   51G   28G  65%  /winblows
tmpfs                                        3.2G   3.2G    0G 100%  /tmp

However, du showed the directory to be empty:

$ du -sch /tmp/
0   /tmp/
0   total

Running rm -rf /tmp made no difference either. All this occupied space was freed when I closed the terminal that was running the loop so I guess that it was taken up by the scrollback information of the terminal.

I know that tmpfs is a storage facility designed to mimic a filesystem. OK but

  1. How does this work? Is /tmp a file somewhere (yes, I know that "everything is a file"), I mean is it a "normal" file stored somewhere on the hard drive? Is it a part of my swap partition? Of my RAM? If I understand correctly, it can be either in RAM or SWAP depending on current usage but I saw no spikes in RAM or SWAP usage while /tmp was full.

  2. Where/how is the scrollback information from the terminal kept? Since there were no files in /tmp it cannot be stored in a simple text file, so where is it and how come it is taking up space on /tmp?

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up vote 4 down vote accepted

I’m not specifically expert in tmpfs, and I’ve never heard of your terminator terminal emulator, but I can address the general question.  This is an old Unix trick: create a file (commonly, but not necessarily, in /tmp) and then promptly delete it.  As long as you keep it open, it continues to exist, and resides on the filesystem (be it disk, memory, local, or remote), and you can write to it and read from it, just like any ordinary file1, until you close it.  Then it is really deleted.

1 Except for the fact that it doesn’t appear in any directory.  You can think of it as an ordinary file that happens to have zero hard links to it.

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I see, am I correct in assuming that this is similar to how named pipes work? – terdon Feb 1 '13 at 18:08
More like the way unnamed pipes work. – Scott Feb 1 '13 at 18:25

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