Development/Tutorials/Debugging/Debugging IOSlaves/Debugging kio fish

This page is a starting point for debugging kio_fish. Please also read how to debug IO slaves generically.


When you point your konqueror to fish://user@target, the fish kioslave opens a process that calls the executable ssh. If a password is needed, this question is passed on to you (search for "password" in fish.cpp). Then a perl script is copied over to the target host and executed there. On the target host, the file is named This perl script is compiled into your binary kio_fish.o. If you want to change it sustainably, you will have to change in your source dir and then compile and install the directory kioslave/fish.

The perl script, running on the target computer, is (t)here to execute fish commands like LIST, STAT, WRITE and APPEND. They are sent from fish.cpp running on the source computer.


Interesting bugs to get into the topic

Test case

If you want a test case to get some debugging output from kio_fish, get a fresh checkout of kdebase. There you find a test case in runtime/kioslave/fish/tests. I had the problem that the STOR commands from the fish kioslave were not logged as long as I had localhost as target computer. When I changed it to my local address (, it worked.


You may want to switch on logging for fish.cpp and for

In this example, we want to log the output of fish.cpp to /tmp/debugfish. As described at Debugging_IOSlaves, change your kdebugrc, e.g. like that:

cat >> $(kde4-config --path config | sed "s/.*://")kdebugrc << EOF

In this example, we want to log the output of to /tmp/kio_fish.debug.log so we change the 14th line of to


and uncomment all the lines containing DEBUG.

Manually testing fish

To test fish manually, call ~/ The fish server will ask you in friendly words (it says ### 100) to transmit its own code, followed by a line __END__. Transmit it. Then the fish server is ready to talk with you (it says ### 200). You can have a conversation like this:

itchy:~ # (echo __END__; cat)| cat -  | perl
### 100 transfer fish server
### 200
VER 0.0.3 copy lscount lslinks lsmime exec stat
### 200
#WRITE 0 6 /tmp/file1 

### 100
### 200
itchy:~ # cat /tmp/file1
123456itchy:~ #

The WRITE command


#WRITE 0 6 /tmp/file1


#WRITE start end file

Reads the end-start bytes from STDIN and writes them into file file starting at position start ending at position end.

The EXEC command

The EXEC command executes an sh command on the computer where the fish perl service is running.


itchy:~/kdebase/runtime/kioslave/fish # (echo __END__; cat)| cat -  | perl                                           
### 100 transfer fish server
### 200                                       
#EXEC ls blah
### 200
itchy:~/kdebase/runtime/kioslave/fish # cat blah                                                                   

Passwordless login

For your work you may want to setup passwordless logins on localhost. This saves you from typing the password again and again. Here is an example how you can do it.

itchy:~ # ssh-keygen -t dsa
Generating public/private dsa key pair.
Enter file in which to save the key (/root/.ssh/id_dsa):
Enter passphrase (empty for no passphrase):
Enter same passphrase again:
Your identification has been saved in /root/.ssh/id_dsa.
Your public key has been saved in /root/.ssh/
The key fingerprint is:
9c:a3:7e:f8:50:c4:85:2e:97:e5:2f:f7:1e:91:b2:06 [email protected]
The key's randomart image is:
+--[ DSA 1024]----+
|         ..      |
|       ....      |
|       .o+       |
|      .o+..    . |
|       oS E.. o  |
|       o ...oo . |
|      o.   oo..  |
|     ....  .  .. |
|      .o.    ..  |
itchy:~ # cat .ssh/ >> .ssh/authorized_keys
itchy:~ # ssh [email protected]
Last login: Sat Feb 28 17:05:39 2009 from localhost
Have a lot of fun...
itchy:~ #

See also

Content is available under Creative Commons License SA 4.0 unless otherwise noted.