I am completely new to using Gimp, and need to do the following:

open many jpg files as layers (already available)
and then
change the mode of all layers to lighten only

Thanks for your help!

How many layers are you talking? I don't know of a script for that. You might have to do them one at a time.

I think you may try with gmic if you have installed

1 Set "input" to "all layers"
2 Set "output" to new image

In the Gmic menu Layers/lighten ..

It should work and is quick to try

Welcome braphael!
Another option is use Layer Groups.

Oh i almost forgot that now gimp support layer groups

Not sure how they work, how you could set all group to the same layer mode ?

The following script from RobA does it, but it doesn't register in the menus properly (it appears under the Image menu as a change opacity command):

http://silent9.com/incoming/sc ... ttings.scm

Thanks for your help everyone! I will try the script from pointclark first. Or try grouping the layers. There can be up to 25 or 30 layers to lighten at a time, so it would be nice to do it with a script.

I tested the script from pointclark and it works perfectly for my needs. Thanks for all the help!