Saving filtered redis-cli output to a file

I am trying to find when and how the cache is flushed, planned to use the command redis-cli monitor | grep -iE "del|flush" > redis_log.txt for that, but for some reason the file is empty. If i use the command without > redis_log.txt part - it shows a correct output in the terminal, if i use redis-cli monitor > redis_log.txt command - it also saves an actual output to the file, but together it fails, only an empty file is created. Has anybody met a similar issue before?

Solution

As mentioned in the comments, the issue you notice certainly comes from the I/O buffering applied to the grep command, especially when its standard output is not attached to a terminal, but redirected to a file or so.

To be more precise, see e.g. this nice blog article which concludes with this wrap-up:

Here’s how buffering is usually set up:

STDIN is always buffered.

STDERR is never buffered.

if STDOUT is a terminal, line buffering will be automatically selected. Otherwise, block buffering (probably 4096 bytes) will be used.

[…] these 3 points explain all “weird” behaviors.

General solution

To tweak the I/O streams buffering of a program, a very handy program provided by coreutils is stdbuf.

So for your use case:

you may want to replace grep -iE "del|flush" with:
stdbuf -o0 grep -iE "del|flush" to completely disable STDOUT buffering;
or if you'd like some trade-off and just have STDOUT line-buffering,
you may want to replace grep -iE "del|flush" with:
- either stdbuf -oL grep -iE "del|flush",
- or grep --line-buffered -iE "del|flush".

Wrap-up

Finally as suggested by @jetchisel, you'll probably want to redirect STDERR as well to your log file in order not to miss some errors messages… Hence, for example:

redis-cli monitor | stdbuf -o0 grep -iE "del|flush" > redis_log.txt 2>&1