Bash Components

BashShellApp components are probably the easiest to implement and for simple ones it is possible to do all the ‘development’ in EAGLE.

‘Hello World’ in Bash through EAGLE


  • Open EAGLE (e.g. and create a new graph (Graph –> New –> Create New Graph)

  • Drag and drop a ‘Bash Shell App’ from the ‘All Nodes’ palette on the right hand panel onto the canvas.

  • Click on the ‘Bash Shell App’ title bar in the Inspector tab on the right hand panel. This will open all additional settings below.

  • First change the ‘Name’ field of the app in the ‘Display Options’ menu. Call it ‘Hello Word’. Once you leave the entry field also the black title bar will reflect that new name.

  • Now change the description of the app in the ‘Description’ menu. Maybe you write ‘Simple Hello World bash app’.

  • now go down to the ‘Component Parameters’ menu and enter the bash command in the ‘Command’ field:

    echo "Hello World"
  • Now save your new toy graph (Graph –> Local Storage –> Save Graph).

Please note that the Hello World example is also described (with videos) as part of the EAGLE documentation.

That should give you the idea how to use bash commands as DALiuGE components. Seems not a lot? Well, actually this is allowing you to execute whatever can be executed on the command line where the engine is running as part of a DALiuGE graph. That includes all bash commands, but also every other executable available on the PATH of the engine. Now that is a bit more exciting, but the excitement stops as soon as you think about real world (not Hello World) examples: Really useful commands will require inputs and outputs in the form of command line parameters and files or pipes. This is discussed in the Advanced Bash Components chapter.


Do we believe that this is actually really working? Well, probably not. Thus let’s just translate and execute this graph. Note that the graph has neither an input nor an output defined, thus there is nothing you could really expect from running it. However, the DALiuGE engine is pretty verbose when run in debug mode and thus we will use that to investigate what’s happening. The following steps are very helpful when it comes to debugging actual components.

Assuming you have a translator and an engine running you can actually translate and execute this, pretty useless, graph. If you have the engine running locally in development mode, you can even see the output in the session log file:

cd /tmp/dlg/logs
ls -ltra dlg_*

The output of the ls command looks like:

-rw-r--r-- 1 root root 1656 Sep 14 16:46 dlg_172.17.0.3_Diagram-2021-09-14-16-41-283_2021-09-14T08-46-17.341082.log
-rw-r--r-- 1 root root 6991 Sep 14 16:46 dlg_172.17.0.3_Diagram-2021-09-14-16-41-284_2021-09-14T08-46-52.618798.log
-rw-r--r-- 1 root root 6991 Sep 14 16:47 dlg_172.17.03_Diagram-2021-09-14-16-41-284_2021-09-14T08-47-28.890072.log

There could be a lot more lines on top, but the important one is the last line, which is the log-file of the session last executed on the engine. Just dump the content to the screen in a terminal:

cat dlg_172.17.03_Diagram-2021-09-14-16-41-284_2021-09-14T08-47-28.890072.log

Since the engine is running in debugging mode there will be many lines in this file, but towards the end you will find something like:

2021-09-14 08:47:28,912 [ INFO] [      Thread-62] [2021-09-14] dlg.apps.bash_shell_app#_run_bash:217 Finished in 0.006 [s] with exit code 0
2021-09-14 08:47:28,912 [DEBUG] [      Thread-62] [2021-09-14] dlg.apps.bash_shell_app#_run_bash:220 Command finished successfully, output follows:
Hello World

2021-09-14 08:47:28,912 [DEBUG] [      Thread-62] [2021-09-14] dlg.manager.node_manager#handleEvent:65 AppDrop uid=2021-09-14T08:46:48_-1_0, oid=2021-09-14T08:46:48_-1_0 changed to execState 2
2021-09-14 08:47:28,912 [DEBUG] [      Thread-62] [2021-09-14] dlg.manager.node_manager#handleEvent:63 Drop uid=2021-09-14T08:46:48_-1_0, oid=2021-09-14T08:46:48_-1_0 changed to state 2

In addition to the session log file the same information is also contained in the dlgNM.log file in the same directory. That file contains all logs produced by the node manager for all sessions and more, which is usually pretty distracting. However, the name of the session logs are not known before you deploy the session and thus another trick is to monitor the dlgNM.log using the tail command:

tail -f dlgNM.log

When you now deploy the graph again and watch the terminal output, you will see a lot of messages pass through.

Treatment of Command Line Parameters

DALiuGE has multiple ways to pass command line parameters to a bash command. The same feature is also used for Docker, Singularity and MPI components. In order to facilitate this DALiuGE is using a few reserved component parameters:

  • command: The value is used as the command (utility) to be executed. It is possible to specify more complex command lines in this value, but the end-user needs to know the syntax.

  • command_line_arguments: Similar to the command, but the value only contains any additional arguments. Again the end-user needs to know all the details.

  • Arg00 - Arg09: (.. deprecated:: use applicationArgs instead, only kept for backwards compatibility)

  • applicationArgs: This is serialized as separate dictionary in JSON and every applicationParam is one entry where the key is the parameter name. This is the most recent way of defining and passing arguments on the command line and allows the developer to define every single argument in detail, including some description text, default value, write protection and type specification/verification. EAGLE displays the complete set and allows users to spicify and modify the content. DALiuGE will process and concatenate all of the parameters and attach them to the command line. In order to enable the user/developer to control the behaviour of the processing there are two additional parameters:

  • argumentPrefix: The value is prepended to each of the parameter names of the applicationArgs. If not specified, the default is ‘–’. In addition, if argumentPrefix==’–’ and an argument name is only a single character long, the argumentPrefix will be changed to ‘-’. This allows the construction of POSIX compliant option arguments as well as short argument names.

  • paramValueSeparator: The value is used to concatenate the argument name and the value. The default is ‘ ‘ (space). Some utilities are using a syntax like ‘arg=value’. In that case this parameter can be set accordingly.

  • input_redirection: the value will be prepended to the command line (cmd) using ‘cat {value} > ‘.

  • output_redirection: the value will be appended to the command line (cmd) using ‘> {value}’.

Not all of them need to be present in a component, only the ones the component developer wants to offer to the user. In particular the applicationArgs have been introduced to support complex utilties which can feature more than 100 arguments (think about tar). If more than one way of specifying arguments is available to an end-user, they can be used together, but the order in which these parts are concatenated might produce unwanted results. The final command line is constructed in the following way (not including the deprecated ArgXX parameters)::

cat {input_redirection.value} > {command.value} {argumentPrefix.value}{}{paramValueSeparator}{applicationArgs.value} {command_line_arguments.value} > {output_redirection}

The applicationArgs are treated in the order of appearance. After the construction of the command line, any placeholder strings will be replaced with actual values. In particular strings of the form ‘%iX’ (where X is the index number of the inputs of this component), will be replaced with the input URL of the input with that index (counting from 0). Similarly ‘%oX’ will be replaced with the respective output URLs.

Eventually we will also drop support for the command_line_arguments parameters. However, currently the applicationArgs can’t be used to specify positional arguments (just a value) and thus, as a fallback users can still use one the command_line_arguments to achieve that. It should also be noted that for really simple commands, like the one used in the helloWorld example, users can simply specify that in the command parameter directly and ommit all of the others.

Advanced Bash Components