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) 8192 -c: core file size (blocks) 0 -m: resident set size (kb) unlimited -l: locked memory (kb) 64 -p: processes 8040 -n: file descriptors 1024 -v: address space (kb) unlimited -w: locks unlimited -e: scheduling priority 0 -r: real-time priority 0
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