O que essa operação bash faz? [duplicado]

Question

Recentemente deparei com este comando no bash:

cat > filename << EOF

Eu não entendo a parte << EOF . Pesquisando no operador << , só encontrei a aritmética de turno normal. Brincar com isso também não me deu nenhuma idéia.

Qualquer explicação seria apreciada!

    

Answer 1

É here document , descrito em man bash :

   Here Documents

   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:

          <<[-]word
                  here-document
          delimiter

   No parameter and variable 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-docu- ment are not expanded.  If word is unquoted, all
   lines of the here-document are subjected to parameter
   expansion, command substitution, and arithmetic expansion, the
   character sequence \<newline> is ignored, and \ must be used to
   quote the char- acters \, $, 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 fashion.

Exemplo de uso:

$ cat > filename << EOF
> Write this line to filename
> And this line
> And this
> EOF 
$ cat filename
Write this line to filename 
And this line 
And this