Sorry if this reply is coming too late to be helpful. The situation you describe, of printing the text from the form directly into a pipeline of chained commands, could be relatively risk-free, provided that the commands behave reasonably well when presented with untrusted input on stdin.
That is, if "commanda" and "commandb" really are just (fairly robust) stdin-stdout filters - and if the strings to run those commands are fully defined in your code (i.e. do not contain untrusted strings from the web form), then you won't really be exposing any untrusted data as part of a command line.
Obviously, if "commanda" or "commandb" are not robust when given untrusted input (e.g. if they assume line-oriented input but don't know how to handle really long input lines, or they assume ASCII-only input and do unpredictable things with non-ASCII data), then your process is still facing risks, unless you filter the data appropriately before writing it to the pipe.
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