Piping error messages with grep

Question

I have a script that fails because some files are missing.

Running the script and piping it to grep

$ ./adder | grep Error

produces the following output:

Error in <TFile::ReadBuffer>: error reading all requested bytes from file v2.2_V3_194424_194712/output_853.root, got 0 of 300
Error in <TFile::Init>: v2.2_V3_194424_194712/output_853.root not a ROOT file

and similar output with different files

I'd like to extract the root files like v2.2_V3_194424_194712/output_853.root from this output, but doing ./adder | grep Error | grep .root doesn't work.

Why is that?

Answer 1

You need to redirect standard error stream (numeric code: 2) from first command in the pipe to standard output stream (numeric code: 1), like this:

$ ./adder 2>&1 | grep Error

Answer 2

Like piokuc's suggests, combine stderr with stdout. However, I think you are looking a better invocation of grep:

./adder 2>&1 | grep "^Error" | grep -oP '[^ ]*\.root'