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There is a Unix code I saw and don't have enough background to fully dechipher:

bash /dev/fd/3 3<< 'EOF'

What is the meaning of 3 3 above? I know what is an heredoc, what is Bash, a device, and fd, but I don't know what is 3 space 3... When I searched the phrase quoted in Google I found basically nothing.

The code was given here as a special heredoc that allows running scrips in-place (specifically those that already include internal heredocs and comments), without creating a file with suitable permissions...

Can a Unix master please explain what is the 3 3 part? Thank you,

Community
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1 Answers1

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That phrase is a way of passing a here-doc to bash on a file descriptor other than 0. I.e., 3.

The first part of the phrase, base /dev/fd/3, runs bash with file descriptor 3 as its input, and the second part, 3<< EOF, tells the shell to write the here-doc into that file descriptor.

Note that 3<< must be a single token, without spacing.

By doing this, any stdin reading within the here-doc script will read true input (from file descriptor 0), and not the here-doc itself. As I understand it, without doing something like this, the here-doc ends up being stdin for the inner bash, and in that case it cannot read "true input".

Ralph Rönnquist
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  • Thank you for the answer. File descriptor? Rather than 0? I have no idea what these mean and can go read on it right now but I humbly suggest you would add a parenthesis-level description of these two concepts for future readers... –  Nov 17 '16 at 03:55
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    @Benia http://stackoverflow.com/questions/5256599/what-are-file-descriptors-explained-in-simple-terms – muru Nov 17 '16 at 04:00
  • @muru I have read there passionately and discovered how OS's tend to organize process in trees/tables/u's and attach each file entry in the process these integer file descriptors starting from 1. Yet, I still fail to make the association between the number 3 in the following context --- Why is it 3 and not 2 or 1 (if not 0)... I believe that a clarification that could help and any other newcomer to understand the answer right here on this page, without longer learning outside and then coming back. –  Nov 22 '16 at 04:54
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    I think the story is: The "3" comes from that "3<<" allocates that file descriptor for piping the text to, and then the fd3 gets inherited to the child, who reads it. You could use any free number, say "8<<" and would then need to refer to /dev/fd/8 accordingly. – Ralph Rönnquist Nov 22 '16 at 05:08
  • I suggest to anyone which is new to Linux to read in the following session if something didn't make sense above, and then read here again: http://unix.stackexchange.com/questions/358022/is-it-true-to-conclude-that-there-are-4-types-of-output-we-can-reference-to –  Apr 11 '17 at 08:06