Posted By: Anonymous
I needed to write a script to enter multi-line input to a program (
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
END;, inclusive) and pipes it as an input to
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.
<< exactly do?
And is there a man page for it?
This is called heredoc format to provide a string into stdin. See https://en.wikipedia.org/wiki/Here_document#Unix_shells for more details.
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
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
must be used to quote the characters
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.