ulimit命令用来限制系统用户对 shell 资源的访问，常见用法如下：

[root@localhost ~]$ ulimit -a            # 查看当前所有的资源限制
[root@localhost ~]$ ulimit -u 65535      # 限制用户能打开的最大进程数
[root@localhost ~]$ ulimit -SHn 65535    # 限制同一时间最多可打开的文件数

命令参数：

-a     All current limits are reported
-b     The maximum socket buffer size
-c     The maximum size of core files created 
-d     The maximum size of a process's data segment
-e     The maximum scheduling priority ("nice")
-f     The maximum size of files written by the shell and its children
-i     The maximum number of pending signals
-l     The maximum size that may be locked into memory
-m     The maximum resident set size (many systems do not honor this limit)
-n     The maximum number of open file descriptors (most systems do not allow this value to be set)
-p     The pipe size in 512-byte blocks (this may not be set)
-q     The maximum number of bytes in POSIX message queues
-r     The maximum real-time scheduling priority
-s     The maximum stack size
-t     The maximum amount of cpu time in seconds
-u     The maximum number of processes available to a single user 
-v     The maximum amount of virtual memory available to the shell and, on some systems, to its children
-x     The maximum number of file locks 
-T     The maximum number of threads
-S     Set a soft limit for the given resource
-H     Set a hard limit for the given resource