Shell input as variable - c++

Question

I currently have a c++ Linux program that reads a parameter "P" from a file and loads it in RAM for further operations. The file has the following line :
P = 123
I would like the program to take P from shell input instead of the file. I am open to all options, as long as I can manually enter P while connected in SSH.
What I have in mind is something like an input prompt :
sudo myprogram start
enter P value : (I would manually enter "123" here)
Or maybe an argument :
sudo myprogram start 123
It must be simple to do but I do not know how, so any help is greatly appreciated !

Answer 1

If this is the only data that the file has then the file operation is needless.
Simply pass 123 (or whatever) to your C++ program and convert the string into integer.
Assuming you pass the integer as the second argument then:
int p = atoi(argv[2]);
A better option is to use strtol:
char *s, *ptr;
s = argv[1];
int p = strtol(s, &ptr, 10);
If you can't make changes to the C++ code then simply do:
echo "P = 123" > file && myprogram start
If your file has more content and you can't simply do echo then, replace the existing line with new value:
sed -i "s/P = [0-9]*/P = 123/" file && myprogram start

Answer 2

First version (enter from keyboard):
echo -n "enter P value: "
read P
Second version (pass as shell script argument):
P=$1
Third version (learn bash/shell programming):
Advanced Bash-Scripting Guide
BASH Programming - Introduction HOW-TO
and many others

Answer 3

This is basic C++. Take a look at the sample code below or visit the site I copied it from.
#include <iostream>
// When passing char arrays as parameters they must be pointers
int main(int argc, char* argv[]) {
if (argc < 5) { // Check the value of argc. If not enough parameters have been passed, inform user and exit.
std::cout << "Usage is -in <infile> -out <outdir>\n"; // Inform the user of how to use the program
std::cin.get();
exit(0);
} else { // if we got enough parameters...
char* myFile, myPath, myOutPath;
std::cout << argv[0];
for (int i = 1; i < argc; i++) { /* We will iterate over argv[] to get the parameters stored inside.
* Note that we're starting on 1 because we don't need to know the
* path of the program, which is stored in argv[0] */
if (i + 1 != argc) // Check that we haven't finished parsing already
if (argv[i] == "-f") {
// We know the next argument *should* be the filename:
myFile = argv[i + 1];
} else if (argv[i] == "-p") {
myPath = argv[i + 1];
} else if (argv[i] == "-o") {
myOutPath = argv[i + 1];
} else {
std::cout << "Not enough or invalid arguments, please try again.\n";
Sleep(2000);
exit(0);
}
std::cout << argv[i] << " ";
}
//... some more code
std::cin.get();
return 0;
}
}

Answer 4

Don't you simply want to be prompted, within your C++ program, to input the value?
If that is what you want, this simple code will do the job:
#include <iostream>
int main(int argc, char **argv) {
int p = 0;
std::cout << "Enter P value: ";
std::cin >> p;
std::cout << "Entered value: " << p << std::endl;
return 0;
}