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I publish this question with an answer so to clarify to new users, why some heredoc syntax would deal with file descriptors; I myself had some trouble understanding it at first.

See for example:

bash << EOF0 

    command1
    << E0F1
        Proudly_Printed_With_Heredoc
    EOF1

E0F0

Why should it be something like:

bash /dev/fd/5 5<< 'EOF0'
    command1
    cat << EOF1 >> /etc/apache2/apache2.conf
        Proudly_Printed_With_Heredoc
    EOF1
EOF0

2 Answers2

4

Your first example is missing cat it doesn't need file descriptors.

bash << EOF0 

command1
cat << E0F1
        Proudly_Printed_With_Heredoc
EOF1

E0F0

however file descriptors are needed if you want to interact with some of the commands instead of having them take their input from the heredoc.

Jasen
  • 3,761
0

One needs to have an introduction with the whole concept of "file descriptors" to understand this answer - What is a "file descriptor" and why we need it. If you already have such an introduction, skip to the answer below; don''t --- Please read my answer in this SE thread, and then comeback.

Answer:

In the second way I presented in the question, we pass an heredoc to Bash on a file descriptor other than 0 (in this case - 5:

The first part of the phrase, bash /dev/fd/5, represents a bash with file descriptor 5 as its input, and the second part, 5<< EOF, tells the shell to write the here-doc into that file descriptor.

Notes:

  1. The heredoc hierarchy has nothing to do with the number of the relevant file descriptor.
  2. I gave the number 5 just because it's a nice number far from zero. AFAIK, you could give 50, or 500, or 50,000, and it would work the same way.
Community
  • 1
  • Read more on standard stream which are represented at FD 0,1,2 here: 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 08:02