Question

I want to put my program’s output into a file. I keyed in the following :

./prog > log 2>&1

But there is nothing in the file “log”. I am using the Ubuntu 11.10 and the default shell is bash.

Anybody know the cause of this AND how I can debug this?

Answer 1

There are many possible causes:

• The program reads the input from log file while you try to redirect into it with truncation (see Why doesn't "sort file1 > file1" work?)
• The output is buffered so that you don’t see data in the file until the output buffer is flushed. You can manually call fflush or output std::flush if using C++ I/O stream etc.
• The program is smart enough and disables output if the output stream is not a terminal.
• You look at the wrong file (i.e. in another directory).
• You try to dump file’s contents incorrectly.
• Your program outputs ‘\0’ as the first character so the output appears to be empty, even though there is some data.
• Name your own.

Your best bet is to run this application under a debugger (like gdb) or use strace or ptrace (or both) and see what the program is doing. I mean, really, output redirection works for the last like 40 years, so the problem must be somewhere else.