usage of 'external tool' node

dear knime users,

i wanted to embed some external tools to a more complex knime workflow, but the execution always stops without error message (at about 60-95%) and i have to cancel. to get an idea what is wrong, here the most simple example i could imagine:

file reader: a2f.txt

external node: /bin/grep
commandline arguments: ff

i expected to get the third row as ‘external output’, but the execution does not end :frowning:
any ideas? thanks in advance,