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?
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);
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 );
}
popen definitely does the job that you're looking for, but it has a few drawbacks:
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.
