在工作中，我曾写过一个 Shell 脚本，这个脚本可以从 4 个 NTP 服务器轮流获取时间，然后将最可靠的时间设置为系统时间。
因为我们对于时间的要求比较高，需要在短时间内就获取到正确的时间。所以我们就需要对这个脚本运行时间进行测试，看看从开始运行到正确设置时间需要花费多少时间。
其实在工作中，还有很多情况下需要测试一个脚本或者程序运行多少时间，特别是对于时间性要求比较高的系统更是如此。
对于时间的测试，我们可以用到一个命令：time 。下面我们就详细看看如何使用 time 命令来对脚本/命令进行测时。
1. time 命令基本用法
time 命令最基本的用法，就是 time + 命令 ，比如：
- $ time ping baidu.com
- PING baidu.com (123.125.114.144) 56(84) bytes of data.
- 64 bytes from 123.125.114.144 (123.125.114.144): icmp_seq=1 ttl=56 time=2.83 ms
- 64 bytes from 123.125.114.144 (123.125.114.144): icmp_seq=2 ttl=56 time=2.77 ms
- …………
- ^C
- --- baidu.com ping statistics ---
- 8 packets transmitted, 8 received, 0% packet loss, time 10818ms
- rtt min/avg/max/mdev = 2.765/2.808/2.862/0.039 ms
- real 0m11.173s
- user 0m0.004s
- sys 0m0.002s
在结果里，real 表示从我们执行 ping 命令到最终按 ctrl+c 终止这段时间所耗费的时间;user 及 sys 分别表示 ping 命令在用户空间及内核空间所运行的时间。
2. 将时间信息写入文件
如果我们想把时间信息直接写入到文件，而不是显示在屏幕上，那么我们可以使用 -o 选项，并指定写入的文件路径。
- $ /usr/bin/time -o /home/alvin/time-output.txt ping baidu.com
执行这个命令后，ping 命令的输出结果依然会在终端里，而 time 命令的结果就写入到我们所指定的 time-output.txt 文件里。
-o 选项表示输出文件不存在就创建，如果存在的话就直接覆盖重写。如果我们不想覆盖重写，而是想追加在文件后面，我们可以使用 -a 选项。
- $ /usr/bin/time -a /home/smart/time-output.txt ping linoxide.com
3. 显示更详细的时间信息
time 命令不带选项的话，显示的信息量比较少，如果我们想获得更详细的信息，那么我们可以使用 -v 选项。
- $ /usr/bin/time -v ping baidu.com
- PING baidu.com (123.125.114.144) 56(84) bytes of data.
- 64 bytes from 123.125.114.144 (123.125.114.144): icmp_seq=1 ttl=56 time=2.75 ms
- 64 bytes from 123.125.114.144 (123.125.114.144): icmp_seq=2 ttl=56 time=2.76 ms
- 64 bytes from 123.125.114.144 (123.125.114.144): icmp_seq=3 ttl=56 time=2.85 ms
- 64 bytes from 123.125.114.144 (123.125.114.144): icmp_seq=4 ttl=56 time=2.77 ms
- ^C
- --- baidu.com ping statistics ---
- 4 packets transmitted, 4 received, 0% packet loss, time 3300ms
- rtt min/avg/max/mdev = 2.751/2.785/2.851/0.075 ms
- Command being timed: "ping baidu.com"
- User time (seconds): 0.00
- System time (seconds): 0.00
- Percent of CPU this job got: 0%
- Elapsed (wall clock) time (h:mm:ss or m:ss): 0:03.64
- 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): 2140
- Average resident set size (kbytes): 0
- Major (requiring I/O) page faults: 0
- Minor (reclaiming a frame) page faults: 626
- Voluntary context switches: 10
- Involuntary context switches: 0
- 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
这个结果信息就相当详细了，我们可以获取到足够多我们所需要的信息。
4. 自定义输出格式
默认情况下，time 命令只输出 real，usr，sys 三个内容，如果我们想要个性化一些，算定义它的输出格式，time 命令也是支持的。time 命令支持的格式有很多，如下所示：
- C - Name and command line arguments used
- D - Average size of the process's unshared data area in kilobytes
- E - Elapsed time in a clock format
- F - Number of page faults
- I - Number of file system inputs by the process
- K - Average total memory use of the process in kilobytes
- M - Maximum resident set the size of the process during the lifetime in Kilobytes
- O - Number of file system outputs by the process
- P - Percentage of CPU that the job received
- R - Number of minor or recoverable page faults
- S - Total number of CPU seconds used by the system in kernel mode
- U - Total number of CPU seconds used by user mode
- W - Number of times the process was swapped out of main memory
- X - Average amount of shared text in the process
- Z - System's page size in kilobytes
- c - Number of times the process was context-switched
- e - Elapsed real time used by the process in seconds
- k - Number of signals delivered to the process
- p - Average unshared stack size of the process in kilobytes
- r - Number of socket messages received by the process
- s - Number of socket messages sent by the process
- t - Average resident set size of the process in kilobytes
- w - Number of time the process was context-switched voluntarily
- x - Exit status of the command
如果我们想要输出以下这样的格式：
- Elapsed Time = 0:01:00, Inputs 2, Outputs 1
我们可以这样自定义：
- $ /usr/bin/time -f "Elapsed Time = %E, Inputs %I, Outputs %O" ping baidu.com
- PING baidu.com (220.181.38.148) 56(84) bytes of data.
- 64 bytes from 220.181.38.148 (220.181.38.148): icmp_seq=1 ttl=54 time=1.82 ms
- 64 bytes from 220.181.38.148 (220.181.38.148): icmp_seq=2 ttl=54 time=1.86 ms
- ^C
- --- baidu.com ping statistics ---
- 4 packets transmitted, 4 received, 0% packet loss, time 3003ms
- rtt min/avg/max/mdev = 1.825/1.859/1.879/0.056 ms
- Elapsed Time = 0:03.92, Inputs 0, Outputs 0
如果你想让输出的结果有换行，可以在对应的地方添加 \n ，比如：
- $ /usr/bin/time -f "Elapsed Time = %E \n Inputs %I \n Outputs %O" ping baidu.com
这样输出的结果就类似于这样：
- Elapsed Time = 0:03.92
- Inputs 0
- Outputs 0
