ulimit is a shell builtin like cd, not a separate program. sudo looks for a binary to run, but there is no ulimit binary, which is why you get the error message. You need to run it in a shell.
However, while you do need to be root to raise the limit to 65535, you probably don’t want to run your program as root. So after you raise the limit you should switch back to the current user.
To do this, run:
sudo sh -c "ulimit -n 65535 && exec su $LOGNAME"
and you will get a new shell, without root privileges, but with the raised limit. The exec causes the new shell to replace the process with sudo privileges, so after you exit that shell, you won’t accidentally end up as root again.

sudo 查看ulimit报错：
google 百度，翻译都太拗口，简洁的说：

ulimit是一个shell内置的命令 像cd ,不是一个独立的进程，sudo以二进制方式运行，但是没有ulimit的二进制方式，所以你运行错误。 应该在一个shell中运行。

本文转自    憬薇   51CTO博客，原文链接:http://blog.51cto.com/welcomeweb/2044982