on-the-fly output redirection, seeing the file redirection output while the program is still running

Question

If I use a command like this one:
./program >> a.txt &
, and the program is a long running one then I can only see the output once the program ended. That means I ha

Answer 1

Your problem is that most programs check to see if the output is a terminal or not. If the output is a terminal then output is buffered one line at a time (so each line is output as it is generated) but if the output is not a terminal then the output is buffered in larger chunks (4096 bytes at a time is typical) This behaviour is normal behaviour in the C library (when using printf for example) and also in the C++ library (when using cout for example), so any program written in C or C++ will do this.

Most other scripting languages (like perl, python, etc.) are written in C or C++ and so they have exactly the same buffering behaviour.

The answer above (using LD_PRELOAD) can be made to work on perl or python scripts, since the interpreters are themselves written in C.