here document

A special kind of redirect

Basic format:

command << token
text
token

• command is the name of the command that accepts standard input;
• token is used to indicate the end of the embedded text;
• text text content;

note:

• The token must appear in a separate line, and there is no space at the end of the text line;
• In the here document, quotation marks lose their special meaning;
• <<- Ignore Tab characters in the text;
• << 'token'You can ignore the command substitution, so that it does not perform any transfer;

Example 1: Ignore tabs

cat <<- HELP

	该文件目的在于对镜像进行tar包转移。操作方法
	save.sh [appname1]-[version1] [appname2]-[version2]
	appname   -----   为镜像名字中的模块名称；
	version   -----   为镜像名字中的模块版本号；	
	打包结果为一个以日期为命名的tar包；

	默认镜像前缀为：”harbor.io:1180/szlt“，若要修改为其他的请进入文件修改变量”HARBOR_ADDRESS“的默认值

	eg:
	1. 对于镜像harbor.io:1180/aliyun/ucx:ec39fc1be7，使用方式为 save.sh ucx-ec39fc1be7
	2. 对于镜像harbor.io:1180/aliyun/ucx:ec39fc1be7,harbor.io:1180/aliyun/apg:d4fdb86ff0; 
	   使用方式为：save.sh ucx-ec39fc1be7 apg-d4fdb86ff0
	HELP

Example 2: command can be any command that accepts standard input

#!/bin/sh
sqlplus "Sdadmin/admin" <<eof
@SD_P_CreateSeparatetable.prc;
eof

Example 3: Write text, and ignore the effect of quotation marks

[root@ccod4 ~]# echo $USER
root
[root@ccod4 ~]# cat << eof > a.sh
> "hello word,$USER"
> 'hello word,$USER'
> 
> eof
[root@ccod4 ~]# 
[root@ccod4 ~]# 
[root@ccod4 ~]# cat a.sh
"hello word,root"
'hello word,root'

Example 4: Ignore command substitution

[root@ccod4 ~]# cat << 'eof' > a.sh
> "hello word,$USER"
> 'hello word,$USER'
> eof

[root@ccod4 ~]# 
[root@ccod4 ~]# cat a.sh
"hello word,$USER"
'hello word,$USER'

Reference link:
http://c.biancheng.net/view/3109.html