Shell Programming and Scripting

Hello I'm curious about how to get a bash script to rewrite a line of standard output. For example, many programs track their progress by writing percentages on the screen:

Precent Done: 60%

That line gets updated periodically to reflect the status.. My question, is how do we do this, as opposed to writing to a new line every time you want to update the progress?

Hi, You could try something like this... You may need to find a way to enable escape sequences in Your shell if it's not enabled by default. The trick here is to send BCKSPC with echo.

Code:

#!/bin/sh
echo Started program...
echo -n "Progress: 00" 
for x in $(seq -w 1 10); do # or some other loop condition
	sleep 1 #Or some other useful code here...
	echo -n "\b\b${x}"
done
echo

And You will have be wary of any other output produced by Your program, for example, if Your actions corresponding to sleep above actually writes stuff to stdout, You may want to redirect it temporarily to a file, which You can display after the loop is done.

/lakris

Thanks lakris... The way to enable interpreting escape sequences is to use the -e option on echo, so your echo would look like:

Code:

echo -en "\b\b${x}"

And your script will work perfectly.

To be frank, I thought before of this possibilty (i.e. using backspaces), but I thought its rather "hackish" and painful to use in more general scenarios. Like you said, i'll have to be wary of following lines of output, so it will be rather difficult to implement this technique unless you're always writing to the last line. Isn't there a more generic or standard way of changing a line of output?

Hi again,
well I can see several scenarios...
for example if You start a process that unzips a large file... start it in background with &, then if You loop over a condition (until $fsize -eq $expectedfsize, or something) You could regularly check for filesize (with wc -c maybe) and present the progress as a fraction of how much is done.
And when it comes to a terminal/tty, moving the cursor to where You want it is not hackish! It's how You do it, at least thats the way I see it

The are tools/libs such as ncurses that are made for easy manipulation of text screens. One just has to decide how much one need it and which is the path of least resistance.

/Lakris

Hello Again

But you're not simply moving the cursor, you're also erasing characters, thats why I thought its "hackish"..

Fair enough, maybe I'm asking too much from a pure bash script.. Thanks for replying!

NP!
And Yes, bash and echo may seem a little limited in their abilities, but if You really want to go wild there's ANSI too!

I guess in a "real" program You would define an area for a function to write in so instead of deleteing the previous entry, one would overwrite it, but still You would have to make a lot of assumptions on screen size, ie rows/cols, and such.

/Lakris