Isso significa que você deseja encaminhar a entrada assim que vir um caractere não em branco.
awk -v cmd='otherCommand' '
forward {print | cmd; next}
{initial_output = initial_output $0 "\n"}
NF {printf "%s", initial_output | cmd; forward = 1}'
Exemplo:
$ printf '%b\n' ' ' '' '\t' | awk -v cmd='echo START; sed "s/.*/<&>/"' '
forward {print | cmd; next}
{initial_output = initial_output $0 "\n"}
NF {printf "%s", initial_output | cmd; forward = 1}'
$ printf '%b\n' ' ' '' '\t' something | awk -v cmd='echo START; sed "s/.*/<&>/"' '
forward {print | cmd; next}
{initial_output = initial_output $0 "\n"}
NF {printf "%s", initial_output | cmd; forward = 1}'
START
< >
<>
< >
<something>