Often I want to diff the output of two COMMANDS. That is, I want to run command A, then run command B, then diff the output of both of those.

For example:

sort file1 > file1.sorted
sort file2 > file2.sorted
diff file1.sorted file2.sorted

Can this be done without writing the output to a temporary file?

Answer: YES!

The trick is to use Bash's process substitution feature:

diff <(sort file1) <(sort file2)

Bash changes that into:

diff /dev/fd/63 /dev/fd/62

echo diff <(sort file1) <(sort file2)

It does that ^^^ after first running the two sort commands, and sending their output to filehandles 62 and 63.

Want to do more magic with process substitution? See the following examples:

multitee is something else to look into

There's lots of other I/O redirection tricks. [1] [2]