C++ much faster than Bash script writing to text file

There are several reasons to it.

First off, interpreted execution environments (like bash, perl alongside with non-JITed lua and python etc.) are generally much slower than even poorly written compiled programs (C, C++, etc.).

Secondly, note how fragmented your bash code is - it just writes a line to a file, then it writes one more, and so on. Your C++ program, on the other side, performs buffered write - even without your direct efforts to it. You might see how slower will it run if you substitute

myfile << "Writing this to a file Writing this to a file \n";

with

myfile << "Writing this to a file Writing this to a file" << endl;

for more information about how streams are implemented in C++, and why \n is different from endl, see any reference documentation on C++.

Thirdly, as comments prove, your bash script performs open/close of the target file for each line. This implies a significant performance overhead in itself - imagine myfile.open and myfile.close moved inside your loop body!

As others have already pointed out, this is because you are currently opening and closing the file with each line you write in your script (and shell scripts are interpreted while C++ is compiled). You might batch the writes instead and write once, for example

MSG="something"logfile="test.txt"(for i in {1..10000}; do        echo $MSGdone) >> $logfile

Which will write the message 10k times but only open the log once.

Compiled vs. Interpreted Languages

Bash is interpreted while C++ is compiled. Just that makes it a lot faster