why cant I redirect the output from sed to a file

Question

I am trying to run the following command

./someprogram | tee /dev/tty | sed 's/^.\{2\}//' > output_file

But the file is always blank when I go to check it. If I remove > output_file from the end of the command, I am able to see the output from sed without any issues.

Is there any way that I can redirect the output from sed in this command to a file?

Answer 1

Remove output-buffering from sed command using the -u flag and make sure what you want to log isn't on stderr

-u, --unbuffered

  load minimal amounts of data from the input files and flush the output buffers more often

Final command :

./someprogram | tee /dev/tty | sed -u 's/^.\{2\}//' > output_file

This happens with streams (usually a program sending output to stdout during its whole lifetime).

sed / grep and other commands do some buffering in those cases and you have to explicitly disable it to be able to have an output while the program is still running.