刚才想使用ulimit修改用户进程的用户栈的大小，发现busybox里没有这个命令，上google搜索得到如下解释：
"ulimit" is a shell builtin, meaning it's a reserved word interpreted by the shell, not a busybox applet.
It will not appear in the list of busybox applets, but as long as you have sh, your shell scripts will
understand ulimit commands
发现ulimit是shell的内置命令，需要使用下面的方式使用：
busybox sh -c 'ulimit -a'
然后发现，执行完这条命令后，ulimit就可以直接使用了：
[root@vexpress ]# ulimit -a
-f: file size (blocks) unlimited
-t: cpu time (seconds) unlimited
-d: data seg size (kb) unlimited
-s: stack size (kb)
-c: core file size (blocks)
-m: resident set size (kb) unlimited
-l: locked memory (kb)
-p: processes
-n: file descriptors
-v: address space (kb) unlimited
-w: locks unlimited
-e: scheduling priority
-r: real-time priority
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