Show a sliding window of output from a program

Question

I want to run a command (rsync -av) and only display 10 lines of output at any time. I have found a similar post, but I don't want to redirect it to a file.

I want the output to be displayed simultaneously, but only 10 lines at most should be visible. For example: The output has 100 lines, when its finished. At the beginning, it shows the line 1-10. Then 2-11 --> 3-12 --> 4-13 until 91-100

I tried it with:

rsync -av /source /destination | tail -f -n 10

It's something like this:

rsync -av /source /destination | while read i; do echo -en "\r$i"; done

but with 10 lines instead of 1.

Answer 1

I've done such tricks in the build system of an embedded Linux distro. In my case, it was a little different. The build script restricted the scrolling region (via VT100 escape sequences) so that the log was shown in the top N-4 lines of the terminal. The bottom four lines were turned into a static area that was updated with the build progress: what is currently building, percentage progress, and such.

A way achieve what you're looking for is this:

1. Move the cursor to the bottom row of the terminal.
2. Print ten blank lines to scroll away any existing material.
3. Set the scrolling region to the bottom ten lines.
4. Run the command.
5. Reset the scrolling region.

The escape sequences can be found in numerous references.

The following is something I just banged up. It works with bash on an Ubuntu VM I have here. It relies on $(( ... )) arithmetic, and stty supporting -g to save tty settings in a serialized string. I avoided using \e in printf to denote the escape character, which would make this less portable.

We interrogate the number of lines from the terminal because the LINES variable might not be exported. (We could instead parse out the rows parameter from the output of stty -a; then we could avoid the whole dance of putting the tty in raw mode, and obtaining the terminal emulator's response using dd. On the other hand, this method works even if the rows value from the tty driver is incorrect.)

Save this script as, say last10, make it executable and then try for instance last10 find /etc.

#!/bin/bash

# save tty settings
saved_stty=$(stty -g)

restore()
{
  stty $saved_stty
  # reset scrolling region
  printf "\033[1;${rows}r"

  # move to bottom of display
  printf "\033[999;1H"
}

trap restore int term exit

# move to bottom of display
printf "\033[999;1H"

printf "\n\n\n\n\n\n\n\n\n\n"

# Query the actual cursor position
printf "\033[6n"

# read tty response
tty_response=
stty raw isig -echo
while true; do
    char=$(dd bs=1 count=1 2> /dev/null)
    if [ "$char" = "R" ] ; then
        break;
    fi
    tty_response="$tty_response$char"
done
stty $saved_stty

# parse tty_response
get_size()
{
   cols=$3
   rows=$2
}

save_IFS=$IFS
IFS='[;R'
get_size $tty_response
IFS=$save_IFS

# set scrolling region to 10 lines
printf "\033[$((rows-9));${rows}r"

# move to bottom of display
printf "\033[999;1H"

# run command
"$@"

Answer 2

You will need to keep a count so you can now when you have "skipped" the first ten lines:

rsync -av /source /destination | (c=0; while read i ; do if [ $c -ge 10 ]; then echo $c "$i";  fi; ((c++)); done)

the functioning of the part after the pipe:

(c=0; while read i ; do if [ $c -ge 10 ]; then echo $c "$i";  fi; ((c++)); done)

is perhaps more clear if formatted as:

c=0; 
while read i ; do 
   if [ $c -ge 10 ]; then 
      echo $c "$i";  
   fi; 
   ((c++)); 
done