Bash - Here Documents

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

Setting a variable

• With read

read -r -d '' MY_VARIABLE << 'EOF'
Content
With 
Multiple Line
EOF

• With Cat

MY_VARIABLE=$(cat << 'EOF'
Content
With 
Multiple Line
EOF
)

MY_VARIABLE="Line 1
Line 2
  Indented line
Line with \$variables and \"quotes\""

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:

• parameter expansion,
• command substitution,
• arithmetic expansion,
• or pathname expansion

is performed on the first WordDelimiter.

Quoted

If the first wordDelimiter is:

• quoted, the lines in the here-document are not expanded.
• unquoted, all lines of the here-document are subjected to :
  • parameter expansion,
  • command substitution,
  • and arithmetic expansion.

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.