在Linux命令直接执行的命令time是shell的内部命令，相当于 /usr/bin/time -p,

其统计三个参数：

real time：被测试程序的总的执行时间；

user mode  time： 被测试程序的在user mode 下的cpu执行时间；

system mode time： 被测试程序在kernel mode下cpu执行时间。

 

eg： time  ./a.out

[wzb@embedded ~]$ time ./a.out

real    0m0.001s
user    0m0.000s
sys     0m0.001s
[wzb@embedded ~]$

eg: /usr/bin/time -v  ./a.out

[wzb@embedded ~]$ /usr/bin/time -v ./a.out
        Command being timed: "./a.out"
        User time (seconds): 0.00
        System time (seconds): 0.00
        Percent of CPU this job got: ?%
        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): 1248
        Average resident set size (kbytes): 0
        Major (requiring I/O) page faults: 0
        Minor (reclaiming a frame) page faults: 101
        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
[wzb@embedded ~]$

查看两者的具体区别，请查看man手册里的说明：

有个note，对这两者进行清楚的说明如下：

  Note: some shells (e.g., bash(1)) have a  built-in  time  command  that
       provides less functionality than the command described here.  To access
       the real command, you may need to specify its pathname (something  like
       /usr/bin/time).

关于time的man手册如下：

[wzb@embedded ~]$ which time
/usr/bin/time
[wzb@embedded ~]$ whereis time
time: /usr/bin/time /usr/include/time.h /usr/share/man/man1p/time.1p.gz /usr/share/man/man2/time.2.gz /usr/share/man/man3p/time.3p.gz /usr/share/man/man7/time.7.gz /usr/share/man/mann/time.n.gz /usr/share/man/man1/time.1.gz
[wzb@embedded ~]$ whereis -b time
time: /usr/bin/time /usr/include/time.h
[wzb@embedded ~]$ whereis -m time
time: /usr/share/man/man1p/time.1p.gz /usr/share/man/man2/time.2.gz /usr/share/man/man3p/time.3p.gz /usr/share/man/man7/time.7.gz /usr/share/man/mann/time.n.gz /usr/share/man/man1/time.1.gz
[wzb@embedded ~]$ man time
[wzb@embedded ~]$ man time | cat
TIME(1)                                                                TIME(1)



NAME
       time - time a simple command or give resource usage

SYNOPSIS
       time [options] command [arguments...]

DESCRIPTION
       The  time  command  runs  the  specified program command with the given
       arguments.  When command finishes, time writes a  message  to  standard
       error  giving  timing statistics about this program run.  These statis-
       tics consist of (i) the elapsed real time between invocation and termi-
       nation, (ii) the user CPU time (the sum of the tms_utime and tms_cutime
       values in a struct tms as returned by times(2)), and (iii)  the  system
       CPU  time  (the  sum of the tms_stime and tms_cstime values in a struct
       tms as returned by times(2)).

       Note: some shells (e.g., bash(1)) have a  built-in  time  command  that
       provides less functionality than the command described here.  To access
       the real command, you may need to specify its pathname (something  like
       /usr/bin/time).

OPTION
       -p     When in the POSIX locale, use the precise traditional format
                   "real %f\nuser %f\nsys %f\n"
              (with  numbers  in  seconds) where the number of decimals in the
              output for %f is unspecified but is sufficient  to  express  the
              clock tick accuracy, and at least one.

ENVIRONMENT
       The  variables LANG, LC_ALL, LC_CTYPE, LC_MESSAGES, LC_NUMERIC, NLSPATH
       and PATH are used. The last one to search for command.   The  remaining
       ones for the text and formatting of the output.

EXIT STATUS
       If  command was invoked, the exit status is that of command.  Otherwise
       it is 127 if command could not be found, 126 if it could be  found  but
       could  not  be  invoked, and some other non-zero value (1-125) if some-
       thing else went wrong.

SEE ALSO
       times(2)



GNU VERSION
       Below a description of the GNU 1.7 version of time.   Disregarding  the
       name  of  the  utility, GNU makes it output lots of useful information,
       not only about time used, but also on other resources like memory,  I/O
       and  IPC calls (where available).  The output is formatted using a for-
       mat string that can be specified using the -f option or the TIME  envi-
       ronment variable.

       The default format string is
          %Uuser %Ssystem %Eelapsed %PCPU (%Xtext+%Ddata %Mmax)k
          %Iinputs+%Ooutputs (%Fmajor+%Rminor)pagefaults %Wswaps

       When the -p option is given the (portable) output format
          real %e
          user %U
          sys %S
       is used.

   The format string
       The format is interpreted in the usual printf-like way.  Ordinary char-
       acters are directly copied, tab,  newline  and  backslash  are  escaped
       using \t, \n and \\, a percent sign is represented by %%, and otherwise
       % indicates a conversion. The program time will always add  a  trailing
       newline  itself.   The conversions follow. All of those used by tcsh(1)
       are supported.

       Time

       %E     Elapsed real time (in [hours:]minutes:seconds).

       %e     (Not in tcsh.) Elapsed real time (in seconds).

       %S     Total number of CPU-seconds that the  process  spent  in  kernel
              mode.

       %U     Total number of CPU-seconds that the process spent in user mode.

       %P     Percentage of the CPU that this job got, computed as (%U + %S) /
              %E.

       Memory

       %M     Maximum resident set size of the process during its lifetime, in
              Kbytes.

       %t     (Not in tcsh.) Average resident set  size  of  the  process,  in
              Kbytes.

       %K     Average  total  (data+stack+text)  memory use of the process, in
              Kbytes.

       %D     Average size of the process’s unshared data area, in Kbytes.

       %p     (Not in tcsh.) Average size  of  the  process’s  unshared  stack
              space, in Kbytes.

       %X     Average size of the process’s shared text space, in Kbytes.

       %Z     (Not in tcsh.) System’s page size, in bytes.  This is a per-sys-
              tem constant, but varies between systems.

       %F     Number of major page faults that occurred while the process  was
              running.  These are faults where the page has to be read in from
              disk.

       %R     Number of minor, or recoverable, page faults.  These are  faults
              for pages that are not valid but which have not yet been claimed
              by other virtual pages.  Thus the data  in  the  page  is  still
              valid but the system tables must be updated.

       %W     Number of times the process was swapped out of main memory.

       %c     Number  of  times the process was context-switched involuntarily
              (because the time slice expired).

       %w     Number of waits: times that  the  program  was  context-switched
              voluntarily,  for instance while waiting for an I/O operation to
              complete.

       I/O

       %I     Number of file system inputs by the process.

       %O     Number of file system outputs by the process.

       %r     Number of socket messages received by the process.

       %s     Number of socket messages sent by the process.

       %k     Number of signals delivered to the process.

       %C     (Not in tcsh.) Name and command line arguments  of  the  command
              being timed.

       %x     (Not in tcsh.) Exit status of the command.

GNU OPTIONS
       -f FORMAT, --format=FORMAT
              Specify  output format, possibly overriding the format specified
              in the environment variable TIME.

       -p, --portability
              Use the portable output format.

       -o FILE, --output=FILE
              Do not send the results to stderr, but overwrite  the  specified
              file.

       -a, --append
              (Used together with -o.) Do not overwrite but append.

       -v, --verbose
              Give very verbose output about all the program knows about.

GNU STANDARD OPTIONS
       --help Print  a usage message on standard output and exit successfully.

       -V, --version
              Print version information on standard output, then exit success-
              fully.

       --     Terminate option list.

BUGS
       Not  all resources are measured by all versions of Unix, so some of the
       values might be reported as zero.  The  present  selection  was  mostly
       inspired by the data provided by 4.2 or 4.3BSD.

       GNU time version 1.7 is not yet localized.  Thus, it does not implement
       the POSIX requirements.

       The environment variable TIME was badly chosen.  It is not unusual  for
       systems  like  autoconf  or  make to use environment variables with the
       name of a utility to override the utility to be used. Uses like MORE or
       TIME  for  options  to  programs (instead of program pathnames) tend to
       lead to difficulties.

       It seems unfortunate that -o overwrites instead of appends.  (That  is,
       the -a option should be the default.)

       Mail suggestions and bug reports for GNU time to
       bug-utils@prep.ai.mit.edu
       Please include the version of time, which you can get by running
       time --version
       and the operating system and C compiler you used.

SEE ALSO
       tcsh(1), times(2), wait3(2)

AUTHORS
       David Keppel
              Original version

       David MacKenzie
              POSIXization,  autoconfiscation, GNU getoptization, docu-
              mentation, other bug fixes and improvements.

       Arne Henrik Juul
              Helped with portability

       Francois Pinard
              Helped with portability



                                  2000-12-11                           TIME(1)
[wzb@embedded ~]$