How to identify what is printing on screen in linux? - bash

Question

I'm using a library in my C++ application and trying to capture all the output in a file. I tried to redirect the stderr to stdout and then stdout to a file like so:
./a.out 2>&1 > out.txt
This captures pretty much everything in my application but there are still some output on the console related to the library I'm using. My question is:
Are there anything besides stdout/stderr? (other than stdin)
If there are, how can I identify these in my case?
And then how can I redirect these to the same file?
Note: In case someone is familiar, library is called SystemC (which is an event driven simulation library/language on top of C++ for mainly system/hardware design).

Answer 1

You must set output file before any stream-to-stream redirection, else bash can't detect file name to output. In your case you can see stderr output.
See bash redirections reference manual.
Solution:
./a.out >out.txt 2>&1
Or just:
./a.out &>out.txt

Answer 2

Hmm, well I think what might be happening is that your program is printing to the controlling terminal. One possibility could be to have your program run as a daemon with no controlling terminal. I have a C function that I call to turn my code into a daemon, I got this from a book called The Linux Programming Interface which I highly recommend.
#define BD_NO_CHDIR 01 /* Don't chdir("/") */
#define BD_NO_CLOSE_FILES 02 /* Don't close all open files */
#define BD_NO_REOPEN_STD_FDS 04 /* Don't reopen stdin, stdout, and
stderr to /dev/null */
#define BD_NO_UMASK0 010 /* Don't do a umask(0) */
#define BD_MAX_CLOSE 8192 /* Maximum file descriptors to close if
sysconf(_SC_OPEN_MAX) is indeterminate */
int becomeDaemon(int flags){
int maxfd, fd, new_stdout;
switch (fork()) { /* Become background process */
case -1: return -1;
case 0: break; /* Child falls through... */
default: _exit(EXIT_SUCCESS); /* while parent terminates */
}
if (setsid() == -1) /* Become leader of new session */
return -1;
switch (fork()) { /* Ensure we are not session leader */
case -1: return -1;
case 0: break;
default: _exit(EXIT_SUCCESS);
}
if (!(flags & BD_NO_UMASK0))
umask(0); /* Clear file mode creation mask */
if (!(flags & BD_NO_CHDIR))
chdir("/"); /* Change to root directory */
if (!(flags & BD_NO_CLOSE_FILES)) { /* Close all open files */
maxfd = sysconf(_SC_OPEN_MAX);
if (maxfd == -1) /* Limit is indeterminate... */
maxfd = BD_MAX_CLOSE; /* so take a guess */
for (fd = 0; fd < maxfd; fd++)
close(fd);
}
if (!(flags & BD_NO_REOPEN_STD_FDS)) {
/*
STDIN = 0
STDOUT = 1
STDERR = 2
*/
close(0); /* Reopen standard fd's to /dev/null */
fd = open("/dev/null", O_RDWR);
if (fd != 0) /* 'fd' should be 0 */
return -1;
if (dup2(0, 1) != 1)
return -1;
if (dup2(0, 2) != 2)
return -1;
}
return 0;
}
Now I suppose that you can change the line open("/dev/null", O_RDWR) to open("/home/you/output.txt", O_RDWR) and redirect the output there. Ofcourse then you wouldn't be able to directly input from the terminal to your program, but from the sounds of the error message you're getting I think you're using a socket anyway so could possible write a client to do that for you if it was necessary.
Hope that helps.