--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/src/SystemProcess.h	Sun Nov 10 22:55:42 2019 +0100
@@ -0,0 +1,137 @@
+/**
+ * Relational pipes
+ * Copyright © 2019 František Kučera (Frantovo.cz, GlobalCode.info)
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, version 3 of the License.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program. If not, see <http://www.gnu.org/licenses/>.
+ */
+#pragma once
+
+#include <vector>
+#include <sstream>
+#include <fcntl.h>
+#include <unistd.h>
+#include <sys/wait.h>
+#include <ext/stdio_filebuf.h>
+
+#include <relpipe/writer/typedefs.h>
+#include <relpipe/cli/RelpipeCLIException.h>
+
+namespace relpipe {
+namespace in {
+namespace filesystem {
+
+/**
+ * Simple wrapper for a system process (fork+exec) that captures and returns just the STDOUT.
+ */
+class SystemProcess {
+private:
+	/**
+	 * the command + its arguments
+	 */
+	std::vector<std::string> commandLine;
+	int nullFile = -1;
+
+	/**
+	 * TODO: move to a common library (copied from the AWK module) 
+	 * @param args
+	 */
+	void execp(const std::vector<std::string>& args) {
+		const char** a = new const char*[args.size() + 1];
+		for (size_t i = 0; i < args.size(); i++) a[i] = args[i].c_str();
+		a[args.size()] = nullptr;
+
+		execvp(a[0], (char*const*) a);
+
+		delete[] a;
+		throw relpipe::cli::RelpipeCLIException(L"Unable to do execvp().", relpipe::cli::CLI::EXIT_CODE_UNEXPECTED_ERROR); // TODO: better exception?
+	}
+
+	/**
+	 * TODO: move to a common library (copied from the AWK module) 
+	 * @param readerFD
+	 * @param writerFD
+	 */
+	void createPipe(int& readerFD, int& writerFD) {
+		int fds[2];
+		int result = pipe(fds);
+		readerFD = fds[0];
+		writerFD = fds[1];
+		if (result < 0) throw relpipe::cli::RelpipeCLIException(L"Unable to create a pipe.", relpipe::cli::CLI::EXIT_CODE_UNEXPECTED_ERROR); // TODO: better exception?
+	}
+
+	/**
+	 * TODO: move to a common library (copied from the AWK module) 
+	 */
+	void redirectFD(int oldfd, int newfd) {
+		int result = dup2(oldfd, newfd);
+		if (result < 0) throw relpipe::cli::RelpipeCLIException(L"Unable redirect FD.", relpipe::cli::CLI::EXIT_CODE_UNEXPECTED_ERROR); // TODO: better exception?
+	}
+
+	/**
+	 * TODO: move to a common library (copied from the AWK module) 
+	 */
+	void closeOrThrow(int fd) {
+		int error = close(fd);
+		if (error) throw relpipe::cli::RelpipeCLIException(L"Unable to close FD: " + to_wstring(fd) + L" from PID: " + to_wstring(getpid()), relpipe::cli::CLI::EXIT_CODE_UNEXPECTED_ERROR); // TODO: better exception?
+	}
+
+public:
+
+	SystemProcess(std::vector<std::string> commandLine) : commandLine(commandLine) {
+		nullFile = open("/dev/null", O_RDWR);
+	}
+
+	virtual ~SystemProcess() {
+		close(nullFile);
+	}
+
+	std::string execute() {
+
+		std::stringstream result;
+
+		int hashReaderFD;
+		int hashWriterFD;
+		createPipe(hashReaderFD, hashWriterFD);
+		
+		__pid_t hashPid = fork();
+
+		if (hashPid < 0) {
+			throw relpipe::cli::RelpipeCLIException(L"Unable to fork the hash process.", relpipe::cli::CLI::EXIT_CODE_UNEXPECTED_ERROR); // TODO: better exception?
+		} else if (hashPid == 0) {
+			// Child process
+			closeOrThrow(hashReaderFD);
+			redirectFD(nullFile, STDIN_FILENO);
+			redirectFD(nullFile, STDERR_FILENO);
+			redirectFD(hashWriterFD, STDOUT_FILENO);
+			execp(commandLine);
+		} else {
+			// Parent process
+			closeOrThrow(hashWriterFD);
+
+			__gnu_cxx::stdio_filebuf<char> hashReaderBuffer(hashReaderFD, std::ios::in);
+			std::istream hashReader(&hashReaderBuffer);
+			
+			for (char ch; hashReader.read(&ch, 1).good();) result.put(ch);
+			
+			int waitError;
+			__pid_t waitPID = wait(&waitError);
+			if (waitError) throw relpipe::cli::RelpipeCLIException(L"The child process returned an error exit code.", relpipe::cli::CLI::EXIT_CODE_UNEXPECTED_ERROR); // TODO: better exception?
+		}
+
+		return result.str();
+	}
+};
+
+}
+}
+}