Question

I want to apply a watch command on a mysql query every N seconds, but would like to have the results on the bottom left of the terminal instead of the top left:

watch -n 120 "mysql_query" | column -t"

Shows my results like so:

--------------------------
|xxxxxxxxxxx             |
|xxxxxxxxxxx             |
|xxxxxxxxxxx             |
|                        |
|                        |
--------------------------

Whereas I would like them to have like so:

--------------------------
|                        |
|                        |
|xxxxxxxxxxx             |
|xxxxxxxxxxx             |
|xxxxxxxxxxx             |
--------------------------

Suggestion?

Answer 1

I don’t see a straight-forward way to do this, but I managed to force it to work using the following approach. I haven’t fully tested this so I cannot guarantee that this will work in all situations.

Using this script:

#!/bin/bash
TERM_HEIGHT=`tput lines`  # determine terminal height
WATCH_BANNER_HEIGHT=2  # account for the lines taken up by the header of "watch"
let VIS_LINES="TERM_HEIGHT - WATCH_BANNER_HEIGHT"  # height of visible area
(yes " " | head -n $VIS_LINES; cat | head -n $VIS_LINES) | tail -n $VIS_LINES

Post process the output of your command as it is called by watch e.g. (assuming the script was saved as align_bottom, made executable, and store somewhere within your $PATH):

watch -n 120 "mysql_query | column -t | align_bottom"

What the script does:

1. Determine the height (number of lines) of the terminal
2. Calculate the visible area of the watch output
3. Print blank lines to pad the output (pushing the output down)
4. Read in output from stdin, and trim it so we only show the top of the output if it extends beyond the screen. If you want to see the bottom of the output instead, simple remove the head command after cat.
5. tail the output of steps (3) and (4) so excess padding is removed and the final output fits snugly within watch

I have to admit this seems a little hackish, but hopefully it gets you closer to what you’re trying to achieve.

---

Update:

It should also be possible to implement that as a function instead just so it can sit comfortably in .bashrc.

function align_bottom() {
  (( VIS = $(tput lines) - 2 ))  # height of visible area
  (yes " " | head -n $VIS; cat | head -n $VIS) | tail -n $VIS
}
typeset -fx align_bottom  # !! make it callable from subshell

Usage would be the same:

watch -n 120 "mysql_query | column -t | align_bottom"

Note that watch runs the given command using sh -c, therefore, as Dennis pointed out in the comments, on systems that does not link /bin/sh to /bin/bash the function approach shown above will not work.

It is possible to make it work usign:

watch -n 120 "mysql_query | column -t | bash -c align_bottom"

but for portability and usability, it’s cleaner to simply use the shell script approach.