Q:

Run a simple shell command

What is the best WinAPI function to use when you only want to run a simple shell command like hg > test.txt?

A:

To simply run a file, then ShellExecute() and CreateProcess() are the best options.
As you want to redirect output to a file/run a shell command, it complicates things...
Output redirection is a feature of the command prompt, and as such, the command you want to run needs to be passed to cmd.exe (on NT/XP+) passing /c and your command as the parameters (either ShellExecute or CreateProcess will do).
cmd /c "ipconfig >c:\debug\blah.txt"

The best way however is to use CreateProcess() and create your own pipes to talk to the stdin and stdout of the program (This is all cmd does internally)

