如何使用 libssh 获取错误命令的输出答案

【问题标题】：how to get output of a wrong command with libssh如何使用 libssh 获取错误命令的输出
【发布时间】：2020-12-19 06:36:09
【问题描述】：

我有这个功能，可以通过ssh连接到远程系统：


    std::string _ErrMsg;
    int _RetVal = 0;
    
    MyException errMsg;
    int port = 22;
    try
    {
        if (my_ssh_session == NULL) {
            std::cout << "Error creating ssh session" << std::endl;
            throw MyException("Error in creating session");
            _RetVal = -1;
            return _RetVal;
        }
        ssh_options_set(my_ssh_session, SSH_OPTIONS_HOST, (const void*)(authentication.ip));
        ssh_options_set(my_ssh_session, SSH_OPTIONS_USER, (const void*)(authentication.userName));
        ssh_options_set(my_ssh_session, SSH_OPTIONS_PORT, &port);
        int rc = ssh_connect(my_ssh_session);
        if (rc != SSH_OK) {
            std::cout << "Error with connecting" << std::endl;
            _ErrMsg = ssh_get_error(my_ssh_session);
            ssh_free(my_ssh_session);
            _RetVal = -2;
            throw MyException(_ErrMsg);
        }
        rc = ssh_userauth_password(my_ssh_session, NULL, (const char*)(authentication.pw));
        if (rc != SSH_AUTH_SUCCESS) {
            std::cout << "Authentication failed " << ssh_get_error(my_ssh_session) << std::endl;
            _ErrMsg = ssh_get_error(my_ssh_session);
            ssh_disconnect(my_ssh_session);
            ssh_free(my_ssh_session);
            _RetVal = -3;
            throw MyException(_ErrMsg);
        }
    }
    catch (MyException& e)
    {
        throw e;
    }
    return _RetVal;

还有这个通过ssh channel执行命令的函数：

std::string ssh::exec_ssh_command(char* command)
{
    std::string receive = "";
    std::string err;
    int rc, nbytes;
    char buffer[2000];
    MyException errMsg;
    try {
        my_ssh_session = ssh_new();

        ssh_channel channel = ssh_channel_new(my_ssh_session);
        if (channel == NULL)
        {
            receive = "Channel allocation failed.";
            throw MyException(receive);
        }
        rc = ssh_channel_open_session(channel);
        if (rc != SSH_OK)
        {
            free_channel(channel);
            receive = "Opening session channel failed.";
            throw MyException(receive);
        }

        rc = ssh_channel_request_exec(channel, command);
        if (rc != SSH_OK) {
            receive = "Channel's request executing failed.";
            free_channel(channel);
            throw MyException(receive);
        }
        nbytes = ssh_channel_read(channel, buffer, sizeof(buffer), 0);
        receive = buffer;
        if (nbytes > 0)
        {
            receive.erase(nbytes - 1, 2000);
        }
        else
        {
            receive = "Error in command: not found or wrong syntax";
            throw MyException(receive);
        }

        if (nbytes < 0)
        {
            receive = "Error in reading data from channel ";
            throw MyException(receive);
        }
        free_channel(channel);
        free_session(my_ssh_session);
    }
    catch (MyException& err)
    {
        throw err;

    }
    return receive;
}

当一个错误的命令被发送到这个函数时，我想抛出一个异常。例如，如果我发送此命令：ls /sys/class/net | se -n - 1p。如果我在终端上运行这个命令，我得到这个错误：se: command not found。当我像这样运行它时：ls /sys/class/net | sed -n -s 1p | grep 'something irrelevant'，它什么也没有。它以两种方式都没有提供任何输出。 nbytes 将是0。有什么办法可以接受se: command not found ??

【问题讨论】：

不相关，考虑使用带有自定义删除功能的std::unique_ptr 以防止泄露您的 ssh_session 和通道。

标签： c++ linux exception libssh

【解决方案1】：

错误打印在 stderr 通道上，这与默认的 stdout 通道不同。

ssh_channel_read 的最后一个参数决定读取哪个：0 表示标准输出，1 表示标准错误。我建议你通过调用ssh_channel_read 两次来阅读这两个频道。

【讨论】：