Issuing System Commands in Linux from C, C++

Issuing system commands in Linux from C, C++

Not surprisingly, the command is still

system("whatever");

and the header is still stdlib.h. That header file's name means "standard library", which means it's on every standard platform that supports C.

And yes, calling system() is often a bad idea. There are usually more programmatic ways of doing things.

If you want to see how lsmod works, you can always look-up its source code and see what the major system calls are that it makes. Then use those calls yourself.

A quick Google search turns up this link, which indicates that lsmod is reading the contents of /proc/modules.

How do I execute a Shell built-in command with a C function?

You should execute sh -c echo $PWD; generally sh -c will execute shell commands.

(In fact, system(foo) is defined as execl("sh", "sh", "-c", foo, NULL) and thus works for shell built-ins.)

If you just want the value of PWD, use getenv, though.

Try to execute command line codes from c++ linux

If you want to run a program and wait for it to finish before executing next line, take a look at this page and example code here: http://www.thegeekstuff.com/2012/03/c-process-control-functions/

But if you want to run gnome-terminal and execute a command in newly created window, do this:

system("gnome-terminal -x sh -c 'cd /tmp ; ls -la'");

How do I execute a command and get the output of the command within C++ using POSIX?

#include <cstdio>
#include <iostream>
#include <memory>
#include <stdexcept>
#include <string>
#include <array>

std::string exec(const char* cmd) {
    std::array<char, 128> buffer;
    std::string result;
    std::unique_ptr<FILE, decltype(&pclose)> pipe(popen(cmd, "r"), pclose);
    if (!pipe) {
        throw std::runtime_error("popen() failed!");
    }
    while (fgets(buffer.data(), buffer.size(), pipe.get()) != nullptr) {
        result += buffer.data();
    }
    return result;
}

Pre-C++11 version:

#include <iostream>
#include <stdexcept>
#include <stdio.h>
#include <string>

std::string exec(const char* cmd) {
    char buffer[128];
    std::string result = "";
    FILE* pipe = popen(cmd, "r");
    if (!pipe) throw std::runtime_error("popen() failed!");
    try {
        while (fgets(buffer, sizeof buffer, pipe) != NULL) {
            result += buffer;
        }
    } catch (...) {
        pclose(pipe);
        throw;
    }
    pclose(pipe);
    return result;
}

Replace popen and pclose with _popen and _pclose for Windows.

How many ways are there to execute system command in C program for windows

Standard C libraries give you only one way to execute external command in OS, so use int system(const char *command).

You can save output of this command to text file, and then read this file from you program.

For example:

#include <stdio.h>
#include <stdlib.h>

#define TMP_FILE_NAME "TMP_FOLDER_CONTENT.txt"

int main(int argc, char *argv[])
{
    system("dir C:\* > "TMP_FILE_NAME);
    FILE * fdir = fopen(TMP_FILE_NAME, "r");
    char buff[100];
    if (fdir)
    {
        while (1) {
            if (fgets(buff, 100, fdir) == NULL) break;
            printf("%s", buff);
        }
    }
    fclose(fdir);
    remove(TMP_FILE_NAME);
    return 0;
}

Where dir is a program to be executed, C:\* - argument of the program, and > - redirection of standard output for that command after which filename TMP_FOLDER_CONTENT.txt will be substituted.

Also you can check returned value, as:

int errorcode = system("dir C:\* > "TMP_FILE_NAME);
printf("Command executed and returned a value %d\n", errorcode);

or taking into account command you use, change the logic of your program, e.g.:

int errorcode = system("dir C:\* > "TMP_FILE_NAME);
if( errorcode )
{
   return errorcode;
}

UPDATE:

Alternatively, you could use pipes in C++, for example as shown in the answer to question How to execute a command and get output of command within C++ using POSIX?

---

Related Topics