Linux

Run rsync, pipe to tee, and redirect the output to a named pipe (mkfifo). Open a second terminal and direct the named pipe into a grep command. Arrange the terminals in whatever way you want.

mkfifo mypipe
rsync | tee mypipe
grep "denied" < mypipe

Tmux is a very helpful terminal multiplexer, meaning it can split your terminal into multiple panes. So, create two side by side panes, then one way of doing it is:

• on the left, run your cmd | tee >(grep 'denied' > error.log)
• on the right, run tail -f error.log

The tee process takes it's standard in, and writes itbto both standard out, so you see all the lines, and the path it's been given. The >(...) operator runs the grep in a subprocess, and returns the path to it's standard input pipe, so grep receives every line, and writes the denied lines to a log file which you display with tail in the other pane.

Rather than using a file for error.log you could also use a named pipe in much the same way.

I daresay there's a way to do something like this with fzf

Given encouragement to try tmux, here is what I've come up with as a "one-liner" (script) that does what I was originally looking for:

#!/bin/sh

tmux new-session -d -s split_screen_grep \; \
  send-keys "/bin/sh -c '$1' | tee /tmp/split_screen_grep.txt" C-m \; \
  split-window -h \; \
  select-pane -t 1 \; \
  send-keys "tail -f /tmp/split_screen_grep.txt | grep '$2'" C-m \;

tmux attach-session -t split_screen_grep

I use it as follows, first arg is a command, second arg is a pattern to search for:

$ ./split-grep "cat big_file.txt" "tmux"

I usually solve this kind of problem by piping to less or a logfile and then just searching in there. You can get it to refresh new content by pressing the End key twice. Or maybe less just needs the -f flag or something similar. I'm too lazy to look it up.

ChatGPT suggests the following:

1. Run tmux
2. rsync -naP --exclude-from=rsync-homedir-local.txt /home/$USER/ $BACKUPDIR/ | tee /tmp/rsync_output.txt
3. Ctrl+B % # splits screen vertically
4. Ctrl+B right-arrow-key # moves to right split
5. tail -f /tmp/rsync_output.txt | grep denied

Not quite a one-liner, but I can see how tmux is a big help here.

Funnily enough Astrogrep on Windows is great for this