为什么进程替换并不总是与在bash while循环工作？
借助进程替换与文件名精细的作品，例如无论
The process substitution works with filenames fine, e.g. both
$ cat <FILENAME和
$ while read i; do echo $i; done <FILENAME工作
但如果不是filename，我们使用回声命令（或任何其他产生输出到标准输出），猫继续工作。
But if instead of FILENAME we use echo command (or any other, which generates output to stdout), cat continues to work
$ cat <(echo XXX)
XXX而循环
$ while read i; do echo $i; done <(echo XXX)
bash: syntax error near unexpected token `<(echo XXX)'
产生错误。
任何想法，为什么？
注意： ＆LT; 的文件名的是的不的进程替换。这是一个重定向。进程替换的格式 ＆LT;（的命令的）
Note: <filename is not process substitution. It's a redirection. Process substitution has the format <(command).
进程替换替换的过程的为＆LT的的名称;（...）。尽管采用的＆LT; 骨节病>符号，它不是一个重定向
Process substitution substitutes the name of a process for the <(...). Despite the use of the < symbol, it is not a redirect.
所以，当你说猫≤（回声富）时，bash创建一个子进程来运行回声命令，并代可以读取以获得该命令的输出的伪文件的名称。替代的结果将是这样的：
So when you say cat <(echo foo), bash creates a subprocess to run the echo command, and substitutes the name of a pseudo-file which can be read to get the output of that command. The result of the substitution will be something like this:
cat /dev/fd/63
请注意没有一个重定向。 （您可以通过键入回声＆LT看到这个动作;（回声富））
Note the absence of a redirect. (You can see this in action by typing echo <(echo foo).)
像许多实用工具，猫可以带或不带命令行参数来调用;如果未指定文件，那么它从标准输入读。因此，猫file.txt的和猫＆LT; file.txt的非常相似。
Like many utilities, cat can be invoked with or without a command-line argument; if no file is specified, then it reads from stdin. So cat file.txt and cat < file.txt are very similar.
但，而命令不接受额外的参数。因此，
But the while command does not accept additional arguments. So
while read -r line; do echo "$line"; done < file.txt
是有效的，但
while read -r line; do echo "$line"; done file.txt
是一个语法错误。
is a syntax error.
进程替换不改变。因此，
Process substitution doesn't change that. So
while read -r line; do echo "$line"; done /dev/fd/63
是一个语法错误，因此所以
is a syntax error, and consequently so is
while read -r line; do echo "$line"; done <(echo foo)
要指定从工艺替代重定向，你需要一个重定向：
To specify the redirect from the process substitution, you need a redirect:
while read -r line; do echo "$line"; done < <(echo foo)
请注意必须有两个℃之间的空间; 骨节病>符号，以避免与这里-doc的语法混乱，＆LT;＆LT;字。
Note that there must be a space between the two < symbols to avoid confusion with the "here-doc" syntax, <<word.