Let us start by installing the library and other tools we need. 


<pre>
<code class="language-bash line-numbers normalize-whitespace">
apt-get update
apt-get install build-essential libgoogle-glog-dev expect-dev tcl8.5 dev
</code>
</pre>



The function below executes a single command and exit after successful ssh login.&nbsp;&nbsp;Please refer to the inline comments and the description that follows for guidance.


<pre>
<code class="language-cpp line-numbers normalize-whitespace">
#include &lt;sys/wait.h>
#include &lt;tcl8.5/expect.h>
#include &lt;glog/logging.h>
#include &lt;iostream>
#include &lt;fstream>
#include &lt;string>
#include &lt;iterator>
#include &lt;sstream>
#include &lt;vector>

using namespace std;

int task(const string& host, const string& password,
 const string& command) {

 exp_is_debugging = 0;
 exp_timeout = 20;

 // exp_open spawn our ssh/program and returns a stream.	
 FILE *expect = exp_popen((char *) host.c_str());
 if (expect == 0) {
 return 1;
 }

 enum { denied, invalid, command_not_found, 
 command_failed, prompt };

 // exp_fexpectl takes variable parameters terminated by
 // exp_end. A parameter takes the form 
 // {type, pattern, and return value} corresponding to
 // exp_glob, "password", prompt, in exp_fexpectl call below.
 // If a pattern matches, exp_fexpectl returns with the value
 // corresponding to the pattern. 

 switch (exp_fexpectl(expect,
 exp_glob, "password: ", prompt,
 exp_end)) {
 case prompt:
 // continue 
 break;
 case EXP_TIMEOUT:
 LOG(ERROR) << "Timeout, may be invalid host" << endl;
 return 1;
 }

 bool success = true

 // In sending command the return character \r is required.
 fprintf(expect, "%s\r", password.c_str());

 switch (exp_fexpectl(expect,
 exp_glob, "denied", denied, // 1 case		
 exp_glob, "invalid password", invalid, // another case
 exp_glob, "$ ", prompt, // third case			
 exp_end)) {
 case denied:
 LOG(ERROR) << "Access denied" << endl;
 success = false;
 break;
 case invalid:
 success = false;
 LOG(ERROR) << "Invalid password" << endl;
 break;
 case EXP_TIMEOUT:
 LOG(ERROR) << "Login timeout" << endl;
 break;
 case prompt:
 // continue;		
 break;
 default:
 break;
 }

 if (!success) {
 fclose(expect);
 waitpid(exp_pid, 0, 0);
 return 1;
 }

 // Note that the pattern used below are dependent
 // on the command. Adding more cases to cover
 // various command will help.

 fprintf(expect, "%s\r", command.c_str());

 switch (exp_fexpectl(expect,
 exp_glob, "cannot create", command_failed,
 exp_glob, "command not found", command_not_found,
 exp_glob, "$ ", prompt,
 exp_end)) {
 case command_failed:
 LOG(ERROR) << "Could not create directory" << endl;
 success = false;;
 break;
 case command_not_found:
 LOG(ERROR) << "Command not found" << endl;
 success = false;
 break;
 case prompt:
 fprintf(expect, "%s\r", "exit");
 LOG(INFO) << "Task completed." << endl;
 break;
 }

 // You should always do this.
 // exp_popen spawns a process with pid exp_pid and 
 // return a FILE object. 

 fclose(expect);
 waitpid(exp_pid, 0, 0);

 if (success)
 return 0;
 else
 return 1;
}
</code>
</pre>

Note that exp_fexpectl will not return and will wait for the pattern to arrive until exp_timeout.&nbsp;&nbsp; Make sure you have all the expected string patterns covered.


The pattern type exp_glob is an enum which determines how the pattern is interpreted.&nbsp;&nbsp;Use exp_regex if you want to use reqular expressions in the case string pattern.&nbsp;&nbsp;Please see expect.h in your include directory for more options.



exp_fexpectv can be used instead of exp_fexpectl. exp_fexpectv takes struct exp_case as parameter.&nbsp;&nbsp;The code fragment below corresponds to the section of code above beginning at line 52.


<pre>
<code class="language-cpp line-numbers normalize-whitespace">
struct exp_case cases [] = { 
 { "denied", NULL, exp_glob, denied},
 { "invalid password", NULL,exp_glob,invalid},
 { "$ ", NULL, exp_glob, prompt},
 0
};

switch (exp_fexpectv (expect, cases)) {	
case denied:
 LOG(ERROR) << "Access denied" << endl;
 retval=error;
 break; 
case invalid:
 retval=failure;
 LOG(ERROR) << "Invalid password" << endl;
 break;
case prompt:
 // continue
 break;
case EXP_TIMEOUT:
 LOG(ERROR) << "Login timeout" << endl;
 break;
}
</code>
</pre>

The terminating null character in exp_case cases above is required.



We have 3 switch blocks above which makes the code a little bit longer.&nbsp;&nbsp;If preferred,&nbsp;all the 3 switch blocks can be placed in a loop and then expand the case patters to cover all possible prompts. 


The main function.


<pre>
<code class="language-cpp line-numbers normalize-whitespace">
int main(int argc, char *argv[]) {
 // Initialize logging
 google::InitGoogleLogging("exp");
 ifstream tasks("tasks.txt", ifstream::in);

 // This is how we read our tasks.txt file. You may
 // want to employ a way you are comfortable with.

 if (tasks.is_open()) {
 for (string line; getline(tasks, line);) {
 stringstream str(line);
 istream_iterator&lt;string> it(str);
 istream_iterator&lt;string> end;
 vector&lt;string> result(it, end);

 string password;
 ostringstream command, host;
 host &lt;< "ssh ";

 for (vector&lt;int>::size_type i = 0;
 i != result.size(); i++) {
 switch (i) {
 case 0:
 host << result [i];
 break;
 case 1:
 password = result [i];
 break;
 default:
 command << result [i] << " ";
 break;
 }
 }

 if (task(host.str(), password, command.str()) == 1) {
 // Error occurred. You may want to stop
 // at this point if the next task is dependent 
 // on the previous task.
 }
 }
 }

 return 0;
}
</code>
</pre>

Our programs takes a text file,&nbsp; tasks.txt as input.&nbsp;&nbsp; Each line is separated by spaces with the first to being use@host and password.&nbsp;&nbsp;The third and beyond is the command and its parameters.



To build the program above use the following from your command line.

<pre>
<code class="language-bash line-numbers normalize-whitespace">
g++ exp.cpp -Wall -std=c++0x -lexpect -ltcl8.5 -lglog -o exp
</code>
</pre>

That's it Good Luck.

Using expect script C/C++ library to manage Linux hosts