Bash语法：什么是“<<”？

Question

Could someone explain the "<<" in the following code?

mysql test<<E0Q
Select * from signins

I'd try to search for it myself, but symbols are hard to search for...

Thanks, Dan

Answer 1

这是一个“here file”，请参阅http://www.linuxjournal.com/article/3771

Answer 2

这里是docs ，或者是一种将大型文本块轻松管理到程序中的方法。

Answer 3

It's not only used for piping though. For example, the scripts used in the Linux From Scratch walkthroughs make extensive use of heredocs in combination with the cat command and the output redirection operator ( > ). Here is an example of such a thing:

user@domain ~$ cat >test.c <<EOF
int main(void){return 0;}
EOF
user@domain ~$

That writes all of the text between the <<EOF start delimiter and the EOF end delimiter to a file `test.c' and once the EOF end delimiter is encountered, you're returned to the shell prompt.

Answer 4

They are called here-documents. From the manual:

Here Documents

 This type of redirection instructs the shell to read input from the current source until a line containing only word (with no trailing blanks) is seen. All of the lines read up to that point are then used as the standard input for a command. The format of here-documents is: <<[-]word here-document delimiter 

Answer 5

If you pipe echo into read , for example, the variable assignment is "lost" since read gets executed in a subshell. You can use a here-doc or a here-string to make it execute in the current shell:

read -a var <<< "some text"

That's an example of another construct you may see called a "here-string", which is similar to a here-doc.

See this and this .