Bash process substitution
"Piping the stdout of a command into the stdin of another is a powerful technique. But, what if you need to pipe the stdout of multiple commands? This is where process substitution comes in.
Process substitution feeds the output of a process (or processes) into the stdin of another process."
The syntax is:
Example: comparing the head of two files using diff
$ diff -u <(head -n3 /var/log/dmesg) <(head -n3 /tmp/dmesg)
--- /proc/self/fd/63 2009-05-26 19:52:45.144544140 +0100
+++ /proc/self/fd/62 2009-05-26 19:52:45.149544007 +0100
@@ -1,3 +1,3 @@
-Initializing cgroup subsys cpuset
-Initializing cgroup subsys cpu
-Linux version 18.104.22.168-170.2.56.fc10.i686 (firstname.lastname@example.org)
(gcc version 4.3.2 20081105 (Red Hat 4.3.2-7) (GCC) )
#1 SMP Mon Mar 23 23:37:54 EDT 2009
+Linux version 22.214.171.124-61.fc6 (email@example.com)
(gcc version 4.1.2 20070626 (Red Hat 4.1.2-13))
#1 SMP Thu Sep 27 18:48:03 EDT 2007
+BIOS-provided physical RAM map:
+ BIOS-e820: 0000000000000000 - 000000000009f000 (usable)
The diff header clearly shows that file descriptors are used as the underlying mechanism.