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);

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If you can’t make changes to the C++ code then simply do:

echo "P = 123" > file && myprogram start 

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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