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.

Your request goes against the unix philosophy. Grep does one thing and does it well. If you desire additional functionality, you should add another utility to accomplish what you want.

rsync -naP --exclude-from=rsync-homedir-local.txt /home/$USER/ $BACKUPDIR/ | grep denied

In your specific task, utilize bashims to do (what I think) you want:

rsync -naP --exclude-from=rsync-homedir-local.txt /home/$USER/ $BACKUPDIR/ || echo "task failed"

Skim has an interactive mode.

Funnily enough Astrogrep on Windows is great for this