How can you diff two pipelines in Bash?

Question

How can you diff two pipelines without using temporary files in Bash? Say you have two command pipelines:

foo | bar
baz | quux

And you want to find the diff in their outputs. One solution would obviously be to:

foo | bar > /tmp/a
baz | quux > /tmp/b
diff /tmp/a /tmp/b

Is it possible to do so without the use of temporary files in Bash? You can get rid of one temporary file by piping in one of the pipelines to diff:

foo | bar > /tmp/a
baz | quux | diff /tmp/a -

But you can't pipe both pipelines into diff simultaneously (not in any obvious manner, at least). Is there some clever trick involving /dev/fd to do this without using temporary files?

Answer 1

A one-line with 2 tmp files (not what you want) would be:

 foo | bar > file1.txt && baz | quux > file2.txt && diff file1.txt file2.txt

With bash, you might try though:

 diff <(foo | bar) <(baz | quux)

 foo | bar | diff - <(baz | quux)  # or only use process substitution once

The 2nd version will more clearly remind you which input was which, by showing
-- /dev/stdin vs. ++ /dev/fd/63 or something, instead of two numbered fds.

---

Not even a named pipe will appear in the filesystem, at least on OSes where bash can implement process substitution by using filenames like /dev/fd/63 to get a filename that the command can open and read from to actually read from an already-open file descriptor that bash set up before exec'ing the command. (i.e. bash uses pipe(2) before fork, and then dup2 to redirect from the output of quux to an input file descriptor for diff, on fd 63.)

On a system with no "magical" /dev/fd or /proc/self/fd, bash might use named pipes to implement process substitution, but it would at least manage them itself, unlike temporary files, and your data wouldn't be written to the filesystem.

You can check how bash implements process substitution with echo <(true) to print the filename instead of reading from it. It prints /dev/fd/63 on a typical Linux system. Or for more details on exactly what system calls bash uses, this command on a Linux system will trace file and file-descriptor system calls

strace -f -efile,desc,clone,execve bash -c '/bin/true | diff -u - <(/bin/true)'

---

Without bash, you could make a named pipe. Use - to tell diff to read one input from STDIN, and use the named pipe as the other:

mkfifo file1_pipe.txt
foo|bar > file1_pipe.txt && baz | quux | diff file1_pipe.txt - && rm file1_pipe.txt

---

Note that you can only pipe one output to multiple inputs with the tee command:

ls *.txt | tee /dev/tty txtlist.txt 

The above command displays the output of ls *.txt to the terminal and outputs it to the text file txtlist.txt.

But with process substitution, you can use tee to feed the same data into multiple pipelines:

cat *.txt | tee >(foo | bar > result1.txt)  >(baz | quux > result2.txt) | foobar