Function captureCommand

programs/docker-init/docker-init.cpp:100–167

Run a command, capture its stdout, return {exit_code, output_lines}.

/// Run a command, capture its stdout, return {exit_code, output_lines}.
std::pair<int, std::vector<std::string>> captureCommand(const std::vector<std::string> & args)
{
    int pipefd[2];
    if (pipe(pipefd) < 0)
        return {-1, {}};

    pid_t pid = fork();
    if (pid < 0)
    {
        (void)close(pipefd[0]);
        (void)close(pipefd[1]);
        return {-1, {}};
    }

    if (pid == 0)
    {
        (void)close(pipefd[0]);
        if (dup2(pipefd[1], STDOUT_FILENO) < 0)
            _exit(127);
        (void)close(pipefd[1]);

        /// Suppress stderr to avoid noise from --try extractions.
        int devnull = open("/dev/null", O_WRONLY);
        if (devnull >= 0)
        {
            (void)dup2(devnull, STDERR_FILENO);
            (void)close(devnull);
        }

        auto argv = buildArgv(args);
        execvp(argv[0], argv.data());
        _exit(127);
    }

    (void)close(pipefd[1]);

    std::string output;
    char buf[4096];
    ssize_t n = 0;
    while ((n = read(pipefd[0], buf, sizeof(buf))) > 0)
        output.append(buf, static_cast<size_t>(n));
    (void)close(pipefd[0]);

    int status = 0;
    while (waitpid(pid, &status, 0) < 0)
        if (errno != EINTR)
            return {-1, {}};

    /// Split output into non-empty lines.
    std::vector<std::string> lines;
    {
        size_t pos = 0;
        while (pos < output.size())
        {
            size_t found = output.find('\n', pos);
            if (found == std::string::npos)
                found = output.size();
            std::string line = output.substr(pos, found - pos);