Am I correct in assuming that "echo" and "cat" are too fast for the "time&q-CodePudding

I was converting a few shell subprocesses to JavaScript in a Node.js project, and out of curiosity I wanted to see the difference between cat and echo with the time command (nothing to do with the project, just wanted to see how fast they were).

time echo "Hello World!"

Which is when I got the following output:

Hello World!

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

Not quite zero but I got a similar output from the cat command:

time cat compile.js

import Build from "./src/build.js";
Build.main();

real    0m0.003s
user    0m0.003s
sys     0m0.000s

Some or all of the time fields say 0m0.000s. Does this mean it's processing so fast that it can't be measured with time? If not, what's happening?

CodePudding user response：

echo (as a shell built-in) probably takes less than 1 millisecond to execute.

Note the rather large real time is due to the inital startup for /bin/echo, which I virtually never use. Subsequent runs are much faster as the executable is cached in memory and doesn't need to be loaded into disk again

cat takes 3 milliseconds, virtually all of which is spent in user-level code, not system-level code. The real time is the same as the user time because, for example, the process is not interrupted to do something else while it is waiting for I/O to complete. On a busier system (or for a longer running command), you may see the real time exceed the sum of the user and system times.

Using /bin/echo instead of the shell built-in can be instructive, because you get very different results for the first execution and subsequent executions:

bash-4.4$ time /bin/echo "Hello World!"
Hello World!
    
real    0m0.113s
user    0m0.001s
sys     0m0.003s

bash-4.4$ time /bin/echo "Hello World!"
Hello World!
    
real    0m0.002s
user    0m0.001s
sys     0m0.001s

I virtually never use /bin/echo, so this is probably the first time I've used it since the last time I rebooted my computer. Much of the 113 milliseconds is spent simply loading the executable into memory, where it gets cached, so it doesn't need to be reloaded for the second execution.