Difference between x | y and y <(x) in bash?

Question

Is there a difference between command1 | command2 and command2 <(command1)?

For example, git diff | more vs more <(git diff)

My understanding is that both take the stdout of command2 and pipe it to the stdin of command1.

Answer 1

The main difference is that <(...), called "process substitution", is translated by the shell into a filename that is passed as a regular argument to the command; it doesn't send anything to the command's standard input. This means that it can't be used directly with commands such as tr which don't take a filename argument:

$ tr a-z A-Z <(echo hello)
usage: tr [-Ccsu] string1 string2
       tr [-Ccu] -d string1
       tr [-Ccu] -s string1
       tr [-Ccu] -ds string1 string2

However, you can always put another < in front of the <(...) to turn it into an input redirection instead:

$ tr a-z A-Z < <(echo hello)
HELLO 

And because it generates a filename, you can use process substitution with commands that take more than one file argument:

$ diff -u <(echo $'foo\nbar\nbaz') <(echo $'foo\nbaz\nzoo')
--- /dev/fd/63  2016-07-15 14:48:52.000000000 -0400
+++ /dev/fd/62  2016-07-15 14:48:52.000000000 -0400
@@ -1,3 +1,3 @@
 foo
-bar
 baz
+zoo

The other significant difference is that a pipe creates subshells which can't have side effects in the parent environment:

 $ echo hello | read x
 $ echo $x
 # nothing - x is not set

But with process substitution, only the process inside the parentheses is in a subshell; the surrounding command can still have side effects:

 $ read x < <(echo hello)
 $ echo $x
 hello

Worth mentioning that you can also write into a process with >(...), although there are fewer cases where that's useful:

$ echo hello > >(cat)
hello