Problem lowering then raising file descriptor limit

[David Flynn]

 If you lower then attempt to raise the file descriptor limit on bash (as a 
non-root user) it won't let you. It should let you raise it back to the level 
it was when the shell started. Csh on the other hand correctly allows you to.

On bash:

dav@chess:~ > ulimit -a
core file size (blocks)     0
data seg size (kbytes)      unlimited
file size (blocks)          unlimited
max locked memory (kbytes)  unlimited
max memory size (kbytes)    unlimited
open files                  1024
pipe size (512 bytes)       8
stack size (kbytes)         unlimited
cpu time (seconds)          unlimited
max user processes          4094
virtual memory (kbytes)     unlimited


dav@chess:~ > ulimit -n 256
dav@chess:~ > ulimit -a


core file size (blocks)     0
data seg size (kbytes)      unlimited
file size (blocks)          unlimited
max locked memory (kbytes)  unlimited
max memory size (kbytes)    unlimited
open files                  256
pipe size (512 bytes)       8
stack size (kbytes)         unlimited
cpu time (seconds)          unlimited
max user processes          4094
virtual memory (kbytes)     unlimited


dav@chess:~ > ulimit -n 1024
bash: ulimit: cannot modify limit: Operation not permitted


 On csh:

chess /home/dav> limit
cputime         unlimited
filesize        unlimited
datasize        unlimited
stacksize       unlimited
coredumpsize    0 kbytes
memoryuse       unlimited
descriptors     1024
memorylocked    unlimited
maxproc         4094
openfiles       1024


chess /home/dav> limit descriptors 256
chess /home/dav> limit


cputime         unlimited
filesize        unlimited
datasize        unlimited
stacksize       unlimited
coredumpsize    0 kbytes
memoryuse       unlimited
descriptors     256
memorylocked    unlimited
maxproc         4094
openfiles       256


chess /home/dav> limit descriptors 1024
chess /home/dav> limit


cputime         unlimited
filesize        unlimited
datasize        unlimited
stacksize       unlimited
coredumpsize    0 kbytes
memoryuse       unlimited
descriptors     1024
memorylocked    unlimited
maxproc         4094
openfiles       1024

                                                        David