On CentOS5, the default limit for open file is 1024. If we want to change the number of open file limit, (say, raise it to 50000), we can do the following:

$ sudo
# vi /etc/security/limits.conf

Enter the following lines to the file /etc/security/limits.conf:

*                hard    nofile          50000
*                soft    nofile          50000

Then save and exit the file. From a regular user's shell, run "ulimit -n", and you will see that the number of open file limit is now "50000":

$ ulimit -n
50000

The soft limit cannot be higher than the number in the hard limit.