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About

Language - Here Document in Bash.

Here document in Bash is implemented through redirection. A 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.

Example

FirstWorldDelimiter not quoted

foo="foo"

cat <<-HereDoc

   my Variable is $foo
   
HereDoc

my Variable is foo

FirstWorldDelimiter quoted

foo="foo"

cat <<-"HereDoc"

   my Variable is $foo
   
HereDoc

my Variable is $foo

Format

The format of here-documents is: 

<<[-]wordDelimiter 
  here-document
wordDelimiter

where:

  • « is the redirection operator
  • - (optional) is used then all leading tab characters are stripped from input lines and the line containing delimiter. This allows here-documents within shell scripts to be indented in a natural fashion.
  • wordDelimiter and wordDelimiter delimit the content of the here-document

Bash variant: The word is expanded and supplied to the command on its standard input. 

<<< word

first WordDelimiter

No:

is performed on the first WordDelimiter.

Quoted

If the first wordDelimiter is:

In the latter case, the character sequence \<newline> is ignored, and \ must be used to quote the characters \, $, and ‘.

second WorldDelimiter

The second WorldDelimiter is generally the result of quote removal on the first WordDelimiter.