Same way as any other command, either of these (or other alternatives, e.g. within "here-documents" or passed as awk variables or...):
printf '%s\n' "$a" | command
command <<<"$a"
How can I use a variable as input in a awk command?
Same way as any other command, either of these (or other alternatives, e.g. within "here-documents" or passed as awk variables or...):
printf '%s\n' "$a" | command
command <<<"$a"