Before a command is executed, its input and output may be redirected using a special notation interpreted
by the shell. Redirection may also be used to open and close files for the current shell execution
environment. The following redirection operators may precede or appear anywhere within a simple command
or may follow a command. Redirections are processed in the order they appear, from left to right.
This type of redirection instructs the shell to read input from the current source until a line
containing only delimiter (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.
The format of here-documents is:
No parameter expansion, command substitution, arithmetic expansion, or pathname expansion is performed on
word. If any characters in word are quoted, the delimiter is the result of quote removal on word, and
the lines in the here-document are not expanded. If word is 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 `.
If the redirection operator is <<-, 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
A variant of here documents, the format is:
The word is expanded and supplied to the command on its standard input.