The here document and the << operator

It is a special type of block of text or code. It is also a special form of I/O redirection. It can be used to feed the command list to an interactive program.

The syntax of the usage of the here document or the << operator is as follows:

command << HERE
text1 …..
text 2….

HERE

This tells the shell that the command should receive the data from a current source, such as the here document, until the pattern is received. In this case, the pattern is HERE. We have used the delimiter as HERE. We can use any other word as the delimiter, such as quite or finish. All the text reads up to a pattern; or the HERE text is used as an input for command. The text or file received by the command is called as the Here document:

$ cat << QUIT
> first input line
> ...
> last input line
> QUIT

The block of text inserted after and before QUIT will be treated as a file. This content will be given as input to the command cat. We will also...