How can I execute external commands in C++/Linux?

I just want to know which is the best way to execute an external command in C++ and how can I grab the output if there is any?

Edit: I Guess I had to tell that I'm a newbie here in this world, so I think I'm gonna need a working example. For example I want to execute a command like:

ls -la

how do I do that?

Solution 1:

Use the popen function.

Example (not complete, production quality code, no error handling):

FILE* file = popen("ls", "r");
// use fscanf to read:
char buffer[100];
fscanf(file, "%100s", buffer);
pclose(file);

Solution 2:

An example:

#include <stdio.h>

int main() {
    FILE * f = popen( "ls -al", "r" );
    if ( f == 0 ) {
        fprintf( stderr, "Could not execute\n" );
        return 1;
    }
    const int BUFSIZE = 1000;
    char buf[ BUFSIZE ];
    while( fgets( buf, BUFSIZE,  f ) ) {
        fprintf( stdout, "%s", buf  );
    }
    pclose( f );
}

Solution 3:

popen definitely does the job that you're looking for, but it has a few drawbacks:

• It invokes a shell on the command you're executing (which means that you need to untaint any user provided command strings)
• It only works in one direction, either you can provide input to the subprocess or you can read its output.

If you want invoke a subprocess and provide input and capture output then you'll have to do something like this:

int Input[2], Output[2];

pipe( Input );
pipe( Output );

if( fork() )
{
    // We're in the parent here.
    // Close the reading end of the input pipe.
    close( Input[ 0 ] );
    // Close the writing end of the output pipe
    close( Output[ 1 ] );

    // Here we can interact with the subprocess.  Write to the subprocesses stdin via Input[ 1 ], and read from the subprocesses stdout via Output[ 0 ].
    ...
}
else
{    // We're in the child here.
     close( Input[ 1 ] );
     dup2( Input[ 0 ], STDIN_FILENO );
     close( Output[ 0 ] );
     dup2( Output[ 1 ], STDOUT_FILENO );

     execlp( "ls", "-la", NULL );
}

Of course, you can replace the execlp with any of the other exec functions as appropriate.