In-Memory Here Doc

When shell encounters any of these syntaxes:

• n<< delimiter or n<<- delimiter

• n<< 'delimiter'or n<<- 'delimiter'

• n<< "delimiter"or n<<- "delimiter"

where n is a number (0-9), it starts creating a multi-line buffer to be sent to file descriptor n.

When you combine that with the exec builtin, you can buffer input to a file descriptor. For example:

1 #!/bin/sh
2 exec 3<< EOF
3 Buffered data
4 EOF
5 cat <&3

Produces:

Buffered data

The above example uses file descriptor 3 because 0-2 are reserved in POSIX for stdin, stdout, and stderr respectively.

An in-memory here-doc can be combined with the indented here-doc.

1 #!/bin/sh
2 if : true; then
3     exec 3<<- EOF
4     Buffered line 1
5     \    Buffered line 2
6     EOF
7     awk 'sub(/^\\\t/,"\t")||1' <&3
8 fi

You can collect the buffered data inside a sub-shell to store it in a variable.

1 #!/bin/sh
2 exec 3<< EOF
3 Buffered data
4 EOF
5 data=$( cat <&3 )
6 echo "$data"

When using the variable to which the data is assigned, quoting it prevents word-splitting from being performed on expansion.