Bash output stream write to a file

Question

so i am running this on bash:

# somedevice -getevent

What this command does is it just keeps running, and everytime my device sends a certa

Answer 1

This is probably because your "somedevice -getevent" command's stdout is being block-buffered. According to this, stdout is by default line-buffered (i.e. what you want) if stdout is a terminal, and block-buffered otherwise.

I'd have a look at the manual for your somedevice command to see if you can force the output to be unbuffered or line-buffered. If not, stdbuf -oL somedevice -getevent > my_record_file should do what you want.

Answer 2

The output is being buffered because the C standard library changes the output buffering mode depending on whether or not stdout is a terminal device. If it's a terminal device (according to isatty(3)), then stdout is line-buffered: it gets flushed every time a newline character gets written. If it's not a terminal device, then it's fully buffered: it only gets flushed whenever a certain amount of data (usually something on the order of 4 KB to 64 KB) gets written.

So, when you redirect the command's output to a file using the shell's > redirection operator, it's no longer outputting to a terminal and it buffers its output. A program can change its buffering mode with setvbuf(3) and friends, but the program has to cooperate to do this. Many programs have command line options to make them line-buffered, e.g. grep(1)'s --line-buffered option. See if your command has a similar option.

If you don't have such an option, you can try using a tool such as unbuffer(1) to unbuffer the output stream, but it doesn't always work and isn't a standard utility, so it's not always available.

Answer 3

You can try 'tee':

somedevice -getevent | tee -a my_record_file

The '-a' option is to append instead of just replacing the content.

Answer 4

The command somedevice probably uses the "Standard Input/Output Library", and in that library, the buffering is on by default. It is switched off when the output does to a terminal/console.

Can you modify the somedevice program? If not, you can still hack around it. See http://www.pixelbeat.org/programming/stdio_buffering/ for details.