how to get linux command output string and output status in c++ - c++

Question

I want to get a Linux command's output string as well as command output status in a C++ program. I am executing Linux commands in my application.
for example:
Command:
rmdir abcd
Command output string:
rmdir: failed to remove `abcd': No such file or directory
Command Status:
1 (Which means command has been failed)
I tried using Linux function system() which gives the output status, and function popen() which gives me output string of a command, but neither function gives me both
the output string and output status of a Linux command.

Answer 1

The output string is in standard output or standard error descriptor (1 or 2, respectively).
You have to redirect these streams (take a look at dup and dup2 function) to a place, where you can read them (for example - a POSIX pipe).
In C I'd do something like this:
int pd[2];
int retValue;
char buffer[MAXBUF] = {0};
pipe(pd);
dup2(pd[1],1);
retValue = system("your command");
read(pd[0], buffer, MAXBUF);
Now, you have (a part of) your output in buffer and the return code in retValue.
Alternatively, you can use a function from exec (i.e. execve) and get the return value with wait or waitpid.
Update: this will redirect only standard output. To redirect standard error, use dup2(pd[1],1).

Answer 2

The simplest solution is to use system, and to redirect standard out and standard error to a temporarly file, which you can delete later.

Answer 3

Unfortunately there's no easy and simple way in C on Linux to do this. Here's an example how to read/write stdout/stderr/stdin of child process correctly.
And when you want to receive exit code you have to use waitpid (complete example is provided on the bottom of the provided page):
endID = waitpid(childID, &status, WNOHANG|WUNTRACED);
Now you just have to join those two together :)
There's also a great free book named Advanced Linux Programming (ALP) containing detailed information about these kinds of problem available here.

Answer 4

Building on Piotr Zierhoffer answer above, here's a function that does just that, and also restores stdout and stderr their original state.
// Execute command <cmd>, put its output (stdout and stderr) in <output>,
// and return its status
int exec_command(string& cmd, string& output) {
// Save original stdout and stderr to enable restoring
int org_stdout = dup(1);
int org_stderr = dup(2);
int pd[2];
pipe(pd);
// Make the read-end of the pipe non blocking, so if the command being
// executed has no output the read() call won't get stuck
int flags = fcntl(pd[0], F_GETFL);
flags |= O_NONBLOCK;
if(fcntl(pd[0], F_SETFL, flags) == -1) {
throw string("fcntl() failed");
}
// Redirect stdout and stderr to the write-end of the pipe
dup2(pd[1], 1);
dup2(pd[1], 2);
int status = system(cmd.c_str());
int buf_size = 1000;
char buf[buf_size];
// Read from read-end of the pipe
long num_bytes = read(pd[0], buf, buf_size);
if(num_bytes > 0) {
output.clear();
output.append(buf, num_bytes);
}
// Restore stdout and stderr and release the org* descriptors
dup2(org_stdout, 1);
dup2(org_stderr, 2);
close(org_stdout);
close(org_stderr);
return status;
}

Answer 5

you can use popen system call, it will redirect output to a file and from file you can redirect output to a string. like :
char buffer[MAXBUF] = {0};
FILE *fd = popen("openssl version -v", "r");
if (NULL == fd)
{
printf("Error in popen");
return;
}
fread(buffer, MAXBUF, 1, fd);
printf("%s",buffer);
pclose(fd);
For more information read man page for popen.