Question

Can command_b knows that command_c is following?

command_a | command_b | command_c

Answer 1

As the other answerers said, no, command_b cannot (need not, should not) know that command_c follows. I’m adding another answer because here is a little additional information:

1. The shell interpreter (Bash, in this case) does, and must know (because it implements the pipe).
2. A program can tell if its standard output is to a terminal. This can be used to change behavior on the fly (for instance, command_b might omit ASCII color codes, while command_c might colorize its output).

See also:

• the isatty(3) manpage
• How to detect if my shell script is running through a pipe?
• Test whether the actual output is a terminal or not in node.js