Count number of bytes piped from one process to another

I'm running a shell script that pipes data from one process to another

process_a | process_b

Does anyone know a way to find out how many bytes were passed between the two programs? The only solution I can think of at the moment would be to write a small c program that reads from stdin, writes to stdout and counts all the of the data transfered, storing the count in an environment variable, like:

process_a | count_bytes | process_b

Does anyone have a neater solution?

Solution 1:

Pipe through dd. dd's default input is stdin and default output is stdout; when it finishes stdin/stdout I/O, it will report to stderr on how much data it transferred.

If you want to capture the output of dd and the other programs already talk to stderr, then use another file-descriptor. Eg,

$ exec 4>~/fred
$ input-command | dd 2>&4 | output-command
$ exec 4>&-

Solution 2:

Use pv the pipe viewer. It's a great tool. Once you know about it you'll never know how you lived without it.

It can also show you a progress bar, and the 'speed' of transfering.