Shell – saving output of another command

catpipeshelltee

I need to select certain lines from the log file and also save them to a text file.
I tried the following but none of them are working as expected. The file "todel.txt" shows 0 bytes.

tail -f general.log | grep Some_word > >(tee -a todel.txt)

tail -f general.log | grep Some_word ; tee  todel.txt

tail -f general.log | grep Some_word | tee -a todel.txt

Best Answer

You need to add stdbuf(1) into your pipeline:

tail -f general.log | stdbuf -oL grep Some_word | tee -a todel.txt

This will set grep's stdout stream buffering mode to be line-buffered, otherwise grep waits to get at least 4096 bytes from the stream (this is the default on Linux for buffered i/o).

Alternatively, you can also call grep with --line-buffered:

tail -f general.log | grep --line-buffered Some_word | tee -a todel.txt

See Turn off buffering in pipe and http://www.pixelbeat.org/programming/stdio_buffering/ for in-detail explanations.

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