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Fixee
Asked: 2011-02-16 12:57:21 +0800 CST

Strange filesystem behavior, Ubuntu 9

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I have two windows open on the same machine (Ubuntu 9, ia32, server). I'll call these windows W1 and W2.

W1: 

$ cd ~/test  
$ ls  
sample  
$

In W2 I run "make" from a parent directory that recreates file test/sample:

$ make project
.
.
$ cd test
$ ls 
sample
$

Now, returning to W1:

$ ls
$ cd ../test
$ ls 
sample
$

In other words, after I build from another window and the file test/sample is replaced, ls shows the file as missing in the 2nd window until I cd ../test back into the directory whereupon it reappears.

I can give more details if required, but just wondering if this is a well-known behavior.

linux filesystems

2 Answers

  1. Best Answer

    Do this experiment, and you'll see what's going on:

    W1: cd /tmp
W1: mkdir foo
W1: cd foo
W1: rmdir ../foo
W1: ls
W1: touch ./bar

W2: cd /tmp
W2: mkdir foo
W2: cd foo
W2: touch bar
W2: ls

W1: ls
W1: touch bar

    Basically, when the working directory a process is in is removed the system can't just randomly change its working directory for it. That would be a huge security hole. So it gives it sort of a fake working directory that can be read (giving 0 bytes) and various other operations, but cannot largely be used. It will happen to have the same name as a new directory created in the same place, but that's superficial.

    If you do lsof | fgrep $$ after the directory is removed you will see an entry like this:

    bash   2924   a_user   cwd   DIR   253,17   48   1327431   /tmp/joe (deleted)
    • 2

  2. You have (resp. your shell has) a file descriptor open to a directory whose nlink count has been reduced to zero (probably — you left out the most important trace of all: from make) — but of course the object lives on as long as a reference is held:

    /dev/shm$ md z
/dev/shm$ cd z
/dev/shm/z$ rd ../z
/dev/shm/z$ ls -al
total 0
/dev/shm/z$ ls -dli .
9347030 drwxr-xr-x 0 me users 40 Feb 15 21:59 .

    Recreating /dev/shm/z creates a new entity.

    /dev/shm/z$ md /dev/shm/z
/dev/shm/z$ ls -dli .
9347030 drwxr-xr-x 0 me users 40 Feb 15 21:59 .
/dev/shm/z$ ls -dli /dev/shm/z
9350877 drwxr-xr-x 2 me users 40 Feb 15 22:03 /dev/shm/z

    All well-defined behavior.

    • 1