kf.kio.gui: "cautious-launcher" "Could not find the program 'cautious-launcher'"

[DinoNerd]

I installed kubuntu 25.10 2 days ago. I haven't done much configuration, but I have installed some programs.

I attempted to launch a java program from the command line, using 'open'


$ open autohaven-xyzzy57.jar
kf.kio.gui: "cautious-launcher" "Could not find the program 'cautious-launcher'"
Could not find the program 'cautious-launcher'

I presume the same thing would happen if I tried to launch it from the file manager - I *think* open is what the file manager uses, but I may be confusing KDE with MacOS'

This looks to me like the handler for java programs in the standard installation is set to use a program that's not part of the standard installation.

I'd guess this is a dependency botch.


There are other ways to launch a java program, and ways to find what package supplies a given program, so I'm not stuck. But if this is a problem with kubuntu's defaults, it ought to be reported somewhere.

[claydoh]

The command open is just a link or alias to xdg-open. More for where there may be multiple applications that can be used, as opposed to a runtime, like java.

So for this, you need to have a java implementation installed, if you do not already.

Then, you should be able to double-click it, or use java --jar nameoffile.jar. Depending on where the terminal is opened, you may need to specify the full path, or cd to its location.

[DinoNerd]

Thank you very much for translating "open" into the actual utility. I'm afraid I'm coming from MacOS, and started by typing what came naturally to me.

Java's installed, and I can run the program with

java -jar autohaven-xyzzy57.jar

My concern was just that it looks as if the default app to handle a java file (configured somewhere I can't find) isn't installed, even with it being the default. This could be designed to make newbie just a bit crazy. (I'm only new to KDE, not linux, so I only got a bit weird ;-))

While I'm whining about trivia - the manpage for xdg-open says is uses "the user's preferred application" and doesn't even hint how they might specify their preference. (I looked in system settings, default applications, and it's all about mime types, not file extensions. It took me a while to figure that searching for java and jar *might* find me mime types that handle java - but even then, "my" preference for java-archive appears to be OpenJDK Java 25 Runtime, with a second choice of Ark. No cautious-launcher in sight.

p.s. My apologies for the font salad in this posting. This UI isn't being cooperative about keeping everything in the same font unless I explicitly request otherwise.

[DinoNerd]

I'm amused to report that cautious-launcher comes from a package called mailcap.

$apt-file find cautious-launcher
mailcap: /usr/bin/cautious-launcher

[claydoh]

My concern was just that it looks as if the default app to handle a java file

Java isn't an *app* in this sense, so xdg-open isn't relevant here like it is for, say, a text file that can have multiple choices installed (Kate, Libreoffice)
Python acts the same way.
I will wager that setting the jar file as executable might just work works with clicking. I don't have any java apps here to verify. . Files normally won't be marked executable ootb for the obvious reasons.

Also note that the output

kf.kio.gui: "cautious-launcher" "Could not find the program 'cautious-launcher'"

This is not an error message; it is informational. It would say 'warning' or 'error' or similar. There is a lot of that sort of thing, sometimes.

and it's all about mime types, not file extensions

That is correct. Linux does not really care about file extensions.