top 默认使用 交互模式
top -b
-b 使用批处理模式
top -b -n 1
使用批处理运行一次

time top -b -n 1
top命令耗时  
real	0m0.159s
user	0m0.006s
sys	0m0.003s

strace top -b -n 1
追踪top运行过程 ，  读取 proc文件

execve("/usr/bin/top", ["top", "-b", "-n", "1"], [/* 22 vars */]) = 0
brk(NULL)                               = 0x1267000
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f6bf162b000
access("/etc/ld.so.preload", R_OK)      = -1 ENOENT (No such file or directory)
open("/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=33846, ...}) = 0
mmap(NULL, 33846, PROT_READ, MAP_PRIVATE, 3, 0) = 0x7f6bf1622000
close(3)                                = 0
...
stat("/proc/2", {st_mode=S_IFDIR|0555, st_size=0, ...}) = 0
open("/proc/2/stat", O_RDONLY)          = 7
read(7, "2 (kthreadd) S 0 0 0 0 -1 213817"..., 1024) = 170
close(7)  


监控指定进程  多个进程id 逗号分隔
top -p pid1,pid2

指定排序字段
top -o %CPU
top -o %MEM
top -o TIME+

 top命令使用