time 不仅仅可以统计时间，还可以反映CPU、内存情况等。
一、time
格式 ： time [options] [commands] [arguments]
如
time ls
real 0m0.016s <== 实际使用时间（real time）
user 0m0.001s <== 用户态使用时间（the process spent in user mode）
sys 0m0.002s <== 内核态使用时间（the process spent in kernel mode）
time命令跟上-p参数可以只打印时间数值（秒数），不打印单位。
如
time –p ls
real 0.01
user 0.00
sys 0.00
二、更为强大的外部命令 /usr/bin/time
其实：
[root@v01 ~]# type -a time
time is a shell keyword
time is /usr/bin/time
time 是/usr/bin/time 一个简单执行
/usr/bin/time 用法
[root@v01 ~]# /usr/bin/time
Usage: /usr/bin/time [-apvV] [-f format] [-o file] [--append] [--verbose]
[--portability] [--format=format] [--output=file] [--version]
[--help] command [arg...]
如
/usr/bin/time ls
0.00user 0.00system 0:00.01elapsed 10%CPU (0avgtext+0avgdata 4080maxresident)k
0inputs+0outputs (0major+294minor)pagefaults 0swaps
很强大有没有啊，再来一发
/usr/bin/time –v ls
Command being timed: "ls"
User time (seconds): 0.00
System time (seconds): 0.00
Percent of CPU this job got: 10%
Elapsed (wall clock) time (h:mm:ss or m:ss): 0:00.01
Average shared text size (kbytes): 0
Average unshared data size (kbytes): 0
Average stack size (kbytes): 0
Average total size (kbytes): 0
Maximum resident set size (kbytes): 4096
Average resident set size (kbytes): 0
Major (requiring I/O) page faults: 0
Minor (reclaiming a frame) page faults: 298
Voluntary context switches: 1
Involuntary context switches: 8
Swaps: 0
File system inputs: 0
File system outputs: 0
Socket messages sent: 0
Socket messages received: 0
Signals delivered: 0
Page size (bytes): 4096
Exit status: 0
重定向到文件 注意格式
{ time command-line; } 2>file
（ time command-line ） 2>file
