Question

I want to make some C++ program and I’m using function popen here to send commands to command line in Unix. It works fine, but when I call cd directory, the directory doesn’t change. I thing that it’s same when I try to run cd directory in some script, after finishing script directory path change back. So, scripts I must run like . ./script.sh not ./sript.sh, but how to do that with popen function? I have tried to add ". " before first argument of popen, but running ". ls" makes error.

Code:

cout << "@ Command from " << session->target().full() << ": " << message.body() << endl;
//cout << "Prisla zprava" << endl;
//m_session->send( "Hello World", "No Subject" );
//system( message.body().c_str() );
//if ( message.body() == "" )
FILE* outp;
char buffer[100];
string outps = "";
outp = popen( message.body().c_str(), "r" );
while ( !feof(outp) )
{
    fgets( buffer, 100, outp );
    outps = outps + buffer;
}
pclose(outp);
cout << "& Output from command: " << outps << endl;
m_session->send( outps.c_str(), "Output" );

In message.body(); is string which I want to run (I’m receiving this from XMPP). When the string is for example "ls", it returns string with list of files in actual directory. But when the message is "cd directory", nothing happens, like trying to change directory in scripts.

Answer 1

Typically, the way the popen() command executes the command is via the shell. So, it opens a pipe, and forks. The child does some plumbing (connecting the pipe to the standard input or standard output – depending on the flag) and then executes

execl("/bin/sh", "/bin/sh", "-c", "what you said", (char *)0);

So, how it all behaves is going to depend on your key environment variables – notably PATH.

If you want to execute a script in the current directory, then one of these options:

outp = popen("./script.sh", "r");
outp = popen("sh -x ./script.sh", "r");
outp = popen("sh -c './script.sh arg1 arg2'", "r");

If you want to execute the ‘ls‘ command:

outp = popen("/bin/ls /the/directory", "r");

And if you want to change directory before running something:

outp = popen("cd /somewhere/else; ./script", "r");

And so on…

If you want to change the directory of the program that is using popen(), then you need to use the ‘chdir()’ system call (or possibly fchdir()). If you think you might want to get back to where you started, use:

int fd = open(".", O_RDONLY);
chdir("/some/where/else");
...do stuff in new directory
fchdir(fd);

(Clearly, you need some error checking in that lot.)