Two "time" in Shell

time is a Linux/Unix external command. It is also a bash keyword. Both are used for measuring time spent on certain process. But there are subtle differences.

The most obvious difference is the output format. But more importantly, as an external command, time takes the program to be run as a parameter. Meanwhile, the builtin time is a modifier of the program (or pipe) to be run. As mentioned in man page:

The use of `time' as a reserved word permits the timing of shell builtins, shell functions, and pipelines. An external `time' command cannot time these easily.

Below are some examples:

• the difference of output format

  [/home/lungangfang/tmp]type -a time
  time is a shell keyword
  time is /usr/bin/time
  [/home/lungangfang/tmp]./a.out
  stdout of program
  stderr of program
  [/home/lungangfang/tmp]time ./a.out
  stdout of program
  stderr of program
  
  real    0m0.001s
  user    0m0.000s
  sys     0m0.000s
  [/home/lungangfang/tmp]/usr/bin/time ./a.out
  stdout of program
  stderr of program
  0.00user 0.01system 0:00.00elapsed ?%CPU (0avgtext+0avgdata 0maxresident)k
  0inputs+0outputs (82major+11minor)pagefaults 0swaps
  

• the difference of IO redirection

  # Built-in time
  [/home/lungangfang/tmp]time ./a.out 2> time.out
  stdout of program
  
  real    0m0.021s
  user    0m0.000s
  sys     0m0.000s
  
  [/home/lungangfang/tmp]cat time.out
  stderr of program
  
  [/home/lungangfang/tmp](time ./a.out) 2> time.out
  stdout of program
  
  [/home/lungangfang/tmp]cat time.out
  stderr of program
  
  real    0m0.001s
  user    0m0.000s
  sys     0m0.000s
  
  # External time
  [/home/lungangfang/tmp]/usr/bin/time ./a.out 2> time.out
  stdout of program
  
  [/home/lungangfang/tmp]cat time.out
  stderr of program
  0.00user 0.00system 0:00.00elapsed ?%CPU (0avgtext+0avgdata 0maxresident)k
  0inputs+0outputs (82major+12minor)pagefaults 0swaps
  
  [/home/lungangfang/tmp]/usr/bin/time -o time.out ./a.out
  stdout of program
  stderr of program
  
  [/home/lungangfang/tmp]cat time.out
  0.00user 0.00system 0:00.00elapsed ?%CPU (0avgtext+0avgdata 0maxresident)k
  0inputs+0outputs (81major+15minor)pagefaults 0swaps