< and << in UNIX /Linux

Question

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

For example

cat << abc.txt

Solution

< 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.

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

Comments

[Seapanther]

Related question: Correct textual name for <<

[S edwards]

I personnaly still don't understand

[devnull]

@Kiwy What is it that you don't understand?

[NoobEditor]

@devnull : so, << will act as input if i say grep "somestring" << filename.txt ...just asking to clear out my head on this!!

[devnull]

@NoobEditor In your example, filename.txt would be treated as a delimiter. Nothing would be read from the file.

[devnull]

@Kiwy I rephrased the answer. Hope it's more clear now.

[S edwards]

I see that. that's better ;-)

[NoobEditor]

@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????

[devnull]

@NoobEditor You might want to refer to a couple of links added in the answer.