"here string"和"here string"之间有什么区别?和回声+管道 [英] What's the difference between "here string" and echo + pipe

问题描述

想知道here-string(here-document)和管道的正确用法是什么.

Wondering what is the right use of here-string (here-document) and pipe.

例如

a='a,b,c,d'
echo $a | IFS=',' read -ra x
IFS=',' read -ra x <<< $a

这两种方法都可以.那么这两个功能之间有什么区别?

Both methods work. Then what would be the difference between the two functionality?

关于阅读"的另一个问题是:

Another problem that I have about "read" is that:

IFS=',' read x1 x2 x3 x4 <<< $a

不起作用，x1的值为"a b c d"，x2，x3，x4的值为

does not work, x1 is valued as "a b c d", and x2, x3, x4 has no value

但是如果:

IFS=',' read x1 x2 x3 x4 <<< "$a"

我可以得到x1 = a，x2 = b，x3 = c，x4 = d一切都好！

I can get x1=a, x2=b, x3=c, x4=d Everything is okay!

有人可以解释吗?

预先感谢

推荐答案

在管道中，创建了两个新进程:一个用于执行echo命令的shell，另一个用于执行read的shell.命令.由于两个子外壳程序都在完成后退出，因此x变量在管道完成后不可用. (在bash 4中，引入了lastpipe选项，以允许管道中的最后一条命令在当前外壳程序(而不是子外壳程序)中执行，从而减轻了此类管道的问题.)

In the pipeline, two new processes are created: one for a shell to execute the echo command, and one for a shell to execute the read command. Since both subshells exit after they complete, the x variable is not available after the pipeline completes. (In bash 4, the lastpipe option was introduced to allow the last command in a pipeline to execute in the current shell, not a subshell, alleviating the problem with such pipelines).

在第二个示例中，here字符串(此处是由单行组成的一种特殊类型的here文档)不需要任何额外的处理，因此x的值实际上是在当前shell中设置的，因此可供稍后在脚本/会话中使用.

In the second example, no extra process is need for the here string (a special type of here document that consists of a single line), so the value of x is in fact set in the current shell, making it available for use later in the script/session.

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