当我在bash中敲入time命令时，运行的其实是bash内置的time命令：

$ time

real    0m0.000s
user    0m0.000s
sys     0m0.000s
$ type time
time is a shell keyword

这个time命令有一个-p选项，表示以posix格式输出：

$ time -p
real 0.00
user 0.00
sys 0.00

除此以外，还有一个time命令。不过我当前的机器并没有安装这个程序：

$ which time
which: no time in (/home/xiaonan/.cargo/bin:/usr/local/sbin:/usr/local/bin:/usr/bin:/opt/cuda/bin:/usr/lib/jvm/default/bin:/usr/bin/site_perl:/usr/bin/vendor_perl:/usr/bin/core_perl)

安装一下，对比bash内置的time命令：

$ sudo pacman -S time
$ type time
time is a shell keyword
$ which time
/usr/bin/time

单独运行“/usr/bin/time -p”，只会输出命令的帮助选项：

$ /usr/bin/time -p
Usage: /usr/bin/time [-apvV] [-f format] [-o file] [--append] [--verbose]
       [--portability] [--format=format] [--output=file] [--version]
       [--help] command [arg...]

需要加上具体的需要度量时间的命令：

$ /usr/bin/time -p echo

real 0.00
user 0.00
sys 0.00

此外也可以给出命令执行的详细信息：

$ /usr/bin/time -v echo

    Command being timed: "echo"
    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): 0:00.00
    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): 1536
    Average resident set size (kbytes): 0
    Major (requiring I/O) page faults: 0
    Minor (reclaiming a frame) page faults: 70
    Voluntary context switches: 1
    Involuntary context switches: 1
    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

参考资料：
/usr/bin/time: not the command you think you know。