Ubuntu – Why does running `time` and `/usr/bin/time` give different results

ps aux includes the full command line (path and parameters), while pgrep only looks at the first 15 characters of the executable's names

ps aux returns the full command line of each process, while pgrep only looks at the names of the executables.

That means that grepping ps aux output will match anything that occurs in the path or the parameters of a process' binary: e.g. `

• ps aux | grep php5 will match /usr/share/php5/i-am-a-perl-script.pl
• but pgrep php5 won't

Take an example from my system -- only we'll use python instead of php5:

• ps aux | grep python gives us:

izx       2348  0.0  0.7 514928 15644 ?        Sl   Jun24   0:00 /usr/bin/python /usr/lib/unity-lens-video/unity-lens-video
izx       2444  0.0  0.9 547392 18864 ?        Sl   Jun24   0:01 /usr/bin/python /usr/lib/unity-scope-video-remote/unity-scope-video-remote
root      2805  0.0  0.5  95436 12204 ?        S    Jun24   0:00 /usr/bin/python /usr/lib/system-service/system-service-d
izx       6272  0.0  2.9 664400 60320 ?        SNl  Jun24   1:16 /usr/bin/python /usr/bin/update-manager --no-focus-on-map
root     11729  0.0  0.9 180508 19516 ?        S    Jun25   0:00 python /usr/lib/software-properties/software-properties-dbus

• But pgrep python returns only 11729, which you'll see from the above list is:

root     11729  0.0  0.9 180508 19516 ?        S    Jun25   0:00 python /usr/lib/software-properties/software-properties-dbus

The problem is that mka is a bash function exported by your script and not an executable (as opposed to make), so /usr/bin/time doesn't find it, as it looks for an executable.

There are two possible solutions.

First, note that there is a difference between the bash built-in function time, and the executable /usr/bin/time. These are different commands, which take different parameters and generate different output:

$ time sleep 1

real    0m1.001s
user    0m0.000s
sys     0m0.001s

$ /usr/bin/time sleep 1
0.00user 0.00system 0:01.00elapsed 0%CPU (0avgtext+0avgdata 664maxresident)k
0inputs+0outputs (0major+215minor)pagefaults 0swaps

Type help time to get help for the bash built-in time. Type man time to get help for the executable /usr/bin/time.

The first solution uses time, whereas the second solution uses /usr/bin/time.

First solution: using the bash built-in function time

The bash built-in function time is aware of declared bash functions, and you can immediately use it to measure the time of such functions.

$ foo() {
    sleep 1;
    echo "Hello World"
  }
$ time foo
Hello World

real    0m1.002s
user    0m0.000s
sys     0m0.001s

In your case, the following command should work:

$ time mka -j8 bacon

This would be my preferred way. However, it may not generate exactly the output you want. In particular, it does not measure or print CPU usage.

Second solution: using the executable /usr/bin/time

You cannot directly measure the time of a built-in bash function using /usr/bin/time (as far as I know), but what you can do is measure the time of the /bin/bash executable executing that bash function. This is a little hacky, and it generates a little overhead as you are launching an extra instance of bash. But this overhead may be negligible when faced with a function that takes minutes to compute, so it may still suit your needs better.

To be able to launch the /bin/bash executable on a bash function, we have to export the function first.

$ foo() {
    sleep 1;
    echo "Hello World"
  }
$ export -f foo
$ echo foo | /usr/bin/time /bin/bash
Hello World
0.00user 0.00system 0:01.00elapsed 0%CPU (0avgtext+0avgdata 1476maxresident)k
0inputs+0outputs (0major+707minor)pagefaults 0swaps

Hence, in your case, to be able to use /usr/bin/time and generate the output format you want, you could proceed as follows:

$ export -f mka
$ echo 'mka -j8 bacon' | /usr/bin/time -f "User\t%U\nSys\t%S\nReal\t%E\nCPU\t%P" /bin/bash