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popen() -like C++ library with iostream support for stdio forwarding

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# cpp-subprocess

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*subprocess* is a small header-only library that provides convenient
C++ API to execute shell commands.  It is similar to `popen()`, as it
creates a new process and executes the command by invoking shell.  The
difference compared to `popen()` is that stdin, stdout and stderr of
the child process are associated to iostream objects which can be
accessed by the parent process.

The library uses GNU extension `__gnu_cxx::stdio_filebuf` to construct
iostream objects from file descriptors.  It works on gcc and it can
work also with clang as long as GNU's C++ standard library is used.

### Tutorial

The first example executes `ls` and print the directory listing of
the current working directory into standard out.

```C++
#include "subprocess.hpp"

int
main(int argc, char *argv[])
{
    subprocess::popen cmd("ls", {});
    std::cout << cmd.out().rdbuf();

return 0;
}
```

The second example executes `sort -r` and write the contents of
`inputfile.txt` to the standard input of the executed command.  It
will then read the standard output of the command and print it.  The
resulting output is the reverse sorted contents of the
`inputfile.txt`.

```C++
#include <fstream>
#include "subprocess.hpp"

int
main(int argc, char *argv[])
{
    subprocess::popen cmd("sort", {"-r"});

std::ifstream file("inputfile.txt");
    std::string line;

while (std::getline(file, line))
    {
       cmd.in() << line << std::endl;
    }
    cmd.close();

std::cout << cmd.out().rdbuf();

return 0;
}
```

Shell-like pipes can be constructed between output and input of two
commands.  Following example executes equivalent of `cat | sort -r`
for input containing three lines: "a", "b" and "c".  The result read
from the output stream contains the input in reversed order.

```C++
#include <fstream>
#include "subprocess.hpp"

int
main(int argc, char *argv[])
{
    subprocess::popen sort_cmd("sort", {"-r"});
    subprocess::popen cat_cmd("cat", {}, sort_cmd.in());

cat_cmd.in() << "a" << std::endl;
    cat_cmd.in() << "b" << std::endl;
    cat_cmd.in() << "c" << std::endl;
    cat_cmd.close();

std::cout << sort_cmd.out().rdbuf();

return 0;
}
```

To write to and read from a subprocess for multiple times, use
`std::getline()` or some other way to read without blocking:

```C++
#include <string>
#include "subprocess.hpp"

int
main(int argc, char *argv[])
{
    std::string buf;

subprocess::popen cat_cmd("cat", {});

cat_cmd.in() << "a" << std::endl;
    std::getline(cat_cmd.out(), buf);
    std::cout << buf << std::endl;

cat_cmd.in() << "b" << std::endl;
    std::getline(cat_cmd.out(), buf);
    std::cout << buf << std::endl;

cat_cmd.in() << "c" << std::endl;
    std::getline(cat_cmd.out(), buf);
    std::cout << buf << std::endl;

return 0;
}
```

### Installation

The library consists of single file `include/subprocess.hpp`.  Copy
the file into your own project or e.g. into `/usr/local/include/`.

Use `-std=c++11` when compiling the code.

Unit test suite depends on Boost Test Library.

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