Run a system command with system() and send output

Question

So what I am trying to do is to invoke a system command with system() function and then whatever its output is I would like to take it and send it over to the client (socket connection).

Client can send various messages. It can be ls but it might be even qwerty. I would like to take the ouput and place it in the write() function as a const void* buffer argument. I have seen this topic but I can get it done to work. So far I thought it could go somewhere of these lines but whatever I tried it did not work.

/* buffer is message from the client: ls, ls -l, whatever*/
system(buffer)
fp = popen(buffer, "r");
if(fp == NULL)
   printf("Failed ot run command\n");

while(fgets(path, sizeof(path), fp) != NULL) {
  //modify output here?
}
pclose(fp);
write(socket_fd, output, strlen(buffer));

It doesn't seem that a command from the command line will also be a file name. — Code-Apprentice, Sep 30 '16 at 20:22
You launch the command twice, once in the call to `system` and once in the call to `popen`. Surely that's not what you wanted to do. — David Schwartz, Sep 30 '16 at 20:23
@Code-Apprentice well I could not find any relevant topic to that beside the one I linked — Lisek, Sep 30 '16 at 20:23
What part of the question that you linked can you not get working? — Code-Apprentice, Sep 30 '16 at 20:24
@Code-Apprentice I can not send it back to client. I can print it but I do not know how to send it back in `write` function — Lisek, Sep 30 '16 at 20:38
I believe the variable `path` contains the output. (This seems like a poor choice of variable names. Rename it to `output` if you wish. Then you can do whatever you want with the data in this buffer, such as send it to `write`. — Code-Apprentice, Sep 30 '16 at 21:10

Code-Apprentice · Accepted Answer · 2016-09-30T21:13:22.090

You should only use popen() and not system() as it describes in the question you linked.

The path variable in the question you linked seems to be misnamed. It contains the output of the system call. You can rename it to output if you wish.

write() takes the length of the buffer you are sending it. In this case, that will be the length of output, not the length of buffer.

Putting this all together gives the following:

char output[1035];
fp = popen(buffer, "r");
if(fp == NULL)
   printf("Failed ot run command\n");

while(fgets(output, sizeof(output), fp) != NULL) {
    write(socket_fd, output, strlen(output));
}
pclose(fp);

Run a system command with system() and send output

1 Answers1