6

I've just come across the << command, used like so:

cat > outfile.txt <<EOF
Multi-line content
that will be written to outfile.txt
EOF

Now, I've no idea what this is called, but I'd quite like to know it's name, primarily so I can go and search and find out more about its syntax. Sadly, Googling for "<<" just doesn't work.

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7
  • Conveniently, I was just trying to figure out how to use it with a stdout redirection last night and couldn't get it. I kept trying to do EOF > outfile.txt at the end, which doesn't work well
    – Michael Mrozek
    Oct 8, 2010 at 14:07
  • 2
    POSIX 2008 > Shell Command Language > Redirection > Here-Document
    – jw013
    Mar 23, 2012 at 5:47
  • 1
    man bash and look for << (in my man viewer, the search command is the / character, so /<< gets me to the right section).
    – jfg956
    Mar 23, 2012 at 8:33
  • For an experiment, you should have tried 'cat <<HERE'. echo doesn't read stdin.
    – XTL
    Mar 23, 2012 at 15:02
  • possible duplicate of How does << work and what is it called?
    – Gilles 'SO- stop being evil'
    Mar 23, 2012 at 18:55

3 Answers 3

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17

That's called a "Here document".

http://en.wikipedia.org/wiki/Here_document

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8

It's form of redirection called a here document or heredoc. It redirects the contents of the given in-line document to a command. The document is delimited by the given word (EOT below). Quoting the word or part of the word after << creates a quoted here-document that the shell will not perform expansions in.

$ tac << EOT
> 123
> 456
> EOT
456
123
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1

In a Unix context it really is known as a "here document." I believe that the "heredoc" construct comes from PHP, Perl, and other scripting languages, and for shell scripting I'd tend to stick with "here document."

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