HELP: Why bash fails to assign a variable when /tmp is full?

[Zhao, Landy (NSB - CN/Qingdao)]

Hello,
I find that when /tmp is full, bash can't assign a value to a variable. Wile it 
still can execute command.
It occurs every time.

Does anyone know the reason?
Can this behavior be changed? (seems no relationship with TMPDIR)

<centos-b:root>/root:
# dd if=/dev/zero of=/tmp/ldt
dd: writing to '/tmp/ldt': No space left on device
1023777+0 records in
1023776+0 records out
524173312 bytes (524 MB) copied, 1.66945 s, 314 MB/s
<centos-b:root>/root:
# df -h /tmp
Filesystem      Size  Used Avail Use% Mounted on
tmpfs           500M  500M     0 100% /tmp
<centos-b:root>/root:
# a=`ls`
-ksh: write to 4 failed [No space left on device]


Thanks,

Landy Zhao
Phone: Jabber
Mobile: +86 15698162651