How does “<<” operator work in linux shell?

Question

I know the following code creates a file core-site.xml in the /opt/hadoop/conf directory. Can some one please break it down in linux shell terms for me? Especially the << operator & CORE_EOF? How does those markers work? I kind of understand this but wanted to know better.

cat >/opt/hadoop/conf/core-site.xml <<CORE_EOF
<?xml version="1.0" encoding="UTF-8"?>
<?xml-stylesheet type="text/xsl" href="configuration.xsl"?>
<configuration>
  <property>
    <name>fs.default.name</name>
    <value>hdfs://localhost:8020</value>
  </property>
</configuration>
CORE_EOF

Answer 1

A command with the << operator will do the following things :

• Launch the program specified in the left of the operator, cat for instance.
• Grab user input, including newlines, until what is specified on the right of the operator is met on one line, EOF for instance

• Send all that have been read except the EOF value to the standard input of the program on the left.

   cat << EOF Hello World EOF 

Will send "Hello

World"

To the standard input of cat.

It is the same as doing this:

cat < file

With file containing :

Hello
World

Answer 2

Cat make a new file or rewrite old with same name in this condition

and put your string into file.

When you want to add strings into file type this:

cat >> /opt/hadoop/conf/core-site.xml << EOF

String

String

EOF