Diff output from two programs without temporary files

Use <(command) to pass one command's output to another program as if it were a file name. Bash pipes the program's output to a pipe and passes a file name like /dev/fd/63 to the outer command.

diff <(./a) <(./b)

Similarly you can use >(command) if you want to pipe something into a command.

This is called "Process Substitution" in Bash's man page.


Adding to both the answers, if you want to see a side by side comparison, use vimdiff:

vimdiff <(./a) <(./b)

Something like this:

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