< and << in UNIX /Linux

Question

What are the operators <<and < used for in Linux?

For example

cat << abc.txt

Related question: Correct textual name for << – Ivan Chau Feb 10 '14 at 09:53 — Ivan Chau, Feb 10 '14 at 09:53

devnull · Accepted Answer · 2014-02-10T09:08:01.373

7

< is used to redirect input. Saying

command < file

executes command with file as input.

The << syntax is referred to as a here document. The string following << is a delimiter indicating the start and end of the here document.

$ cat abc.txt
cat: abc.txt: No such file or directory
$ cat << abc.txt
> Hello!
> Hey :)
> abc.txt
Hello!
Hey :)
$

<< doesn't indicate any sort of indirection.

You might also want to refer to redirection and here document.

edited Feb 10 '14 at 09:08

answered Feb 10 '14 at 08:53

devnull

I personnaly still don't understand – Kiwy Feb 10 '14 at 08:54
@Kiwy What is it that you don't understand? – devnull Feb 10 '14 at 08:55
Well reading it third time I start to understand you should express it as a end of input delimiter instead it would be way more clear than writing some script and explaining after. – Kiwy Feb 10 '14 at 08:57
@devnull : so, << will act as input if i say grep "somestring" << filename.txt ...just asking to clear out my head on this!! – NoobEditor Feb 10 '14 at 08:58
@NoobEditor In your example, filename.txt would be treated as a delimiter. Nothing would be read from the file. – devnull Feb 10 '14 at 09:00
@Kiwy I rephrased the answer. Hope it's more clear now. – devnull Feb 10 '14 at 09:00
I see that. that's better ;-) – Kiwy Feb 10 '14 at 09:01
@devnull : ok..its pretty clear, upvoted...and accepted ( but if some1 else comes up with better answer then u have competition )...can u give me some reference urls where i can see both of these commands in work???? – NoobEditor Feb 10 '14 at 09:06
1

@NoobEditor You might want to refer to a couple of links added in the answer. – devnull Feb 10 '14 at 09:08

1 Answers1