linux shell 命令

在学习PostgreSQL时，有下面一个命令：

$ postgres -D /usr/local/pgsql/data >logfile 2>&1 &

文件描述符1表示标准输出
文件描述符2表述标准错误

File descriptor 1 is the standard output (stdout).
File descriptor 2 is the standard error (stderr).

Here is one way to remember this construct (although it is not entirely accurate): at first, 2>1 may look like a good way to redirect stderr to stdout. However, it will actually be interpreted as “redirect stderr to a file named 1”. & indicates that what follows is a file descriptor and not a filename. So the construct becomes: 2>&1.

参考：
https://stackoverflow.com/questions/818255/in-the-shell-what-does-21-mean

| 是管道符号 表示前面的命令结果作为后面的命令输入

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