Diff output from two programs without temporary files

Question

Say I have too programs a and b that I can run with ./a and ./b.

Is it possible to diff their outputs without first w

Answer 1

Adding a little more to the already good answers (helped me!):

The command docker outputs its help to STD_ERR (i.e. file descriptor 2)

I wanted to see if docker attach and docker attach --help gave the same output

$ docker attach

$ docker attach --help

Having just typed those two commands, I did the following:

$ diff <(!-2 2>&1) <(!! 2>&1)

!! is the same as !-1 which means run the command 1 before this one - the last command

!-2 means run the command two before this one

2>&1 means send file_descriptor 2 output (STD_ERR) to the same place as file_descriptor 1 output (STD_OUT)

Hope this has been of some use.