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linuxbashscriptingheredoc

How does "cat << EOF" work in bash?

I needed to write a script to enter multi-line input to a program (psql).

After a bit of googling, I found the following syntax works:

cat << EOF | psql ---params
BEGIN;

`pg_dump ----something`

update table .... statement ...;

END;
EOF

This correctly constructs the multi-line string (from BEGIN; to END;, inclusive) and pipes it as an input to psql.

But I have no idea how/why it works, can some one please explain?

I'm referring mainly to cat << EOF, I know > outputs to a file, >> appends to a file, < reads input from file.

What does << exactly do?

And is there a man page for it?

Solution

  • This is called heredoc format to provide a string into stdin. See https://en.wikipedia.org/wiki/Here_document#Unix_shells for more details.

    From man bash:

    Here Documents

    This type of 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.

    The format of here-documents is:

              <<[-]word
                  here-document
          delimiter

    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 fashion.