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What does <<< mean? Here is an example:

$ sed 's/a/b/g' <<< "aaa" bbb

Is it something general that works with more Linux commands?

It looks like it's feeding the sed program with the string aaa, but isn't << or < usually used for that?

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    it seems < is for passing file (or directory), << @ for passing multiple lines (similar to the banner command in cisco switches; as terminated by a custom string @ in this case), and <<< to pass a string (instead of file). test them yourself with cat and you'll grasp it very quickly. Commented Oct 5, 2017 at 10:32

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Others have answered the basic question: What is it? (Answer: It's a here string.)

Let's look at why it's useful.

You can also feed a string to a command's stdin like this:

echo "$string" | command

However in Bash, introducing a pipe means the individual commands are run in subshells. Consider this:

echo "hello world" | read first second echo $second $first

The output of the 2nd echo command prints just a single space. Whaaaa? What happened to my variables? Because the read command is in a pipeline, it is run in a subshell. It correctly reads 2 words from its stdin and assigns to the variables. But then the command completes, the subshell exits and the variables are lost.

Sometimes you can work around this with braces:

echo "hello world" | { read first second echo $second $first }

That's OK if your need for the values is contained, but you still don't have those variables in the current shell of your script.

To remedy this confusing situation, use a here string:

read first second <<< "hello world" echo $second $first

Ah, much better!

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    In addition to here-strings, process substutitions are very useful for the same reasons. Commented Jun 22, 2013 at 18:32
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    This is a fantastic explanation of <<< as well as pipes and sub-shells! I learned both from this. Commented May 12, 2017 at 1:29
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    One note, with the simple echo ...|read example, the pipeline can work to set the variables in the current shell if you (1) enable the "lastpipe" shell option (shopt -s lastpipe) and (2) disable job control (set +m) Commented Aug 9, 2017 at 20:00
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    Very interesting. The case you give does not result in unexpected behavior using zsh. Commented May 8, 2019 at 21:00
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    On my zsh, the output for echo $second $first is world hello Commented Jul 25, 2019 at 4:28
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<<< denotes a here string.

$ cat <<< 'hi there' hi there

It passes the word on the right to the standard input of the command on the left.

<< denotes a here document.

$ cat <<EOF > hi > there > EOF hi there

EOF can be any word.

Here documents are commonly used in shell scripts to create whole files or to display long messages.

cat > some-file <<FILE foo bar bar bar foo foo FILE

< passes the contents of a file to a command's standard input.

$ cat < /etc/fstab /dev/sda2 /boot ext4 nosuid,noexec,nodev,rw,noatime,nodiratime 0 2 /dev/sda4 / ext4 rw,noatime,nodiratime, 0 1 /dev/sdb5 /var ext4 nosuid,noexec,nodev,rw,relatime 0 2 ...
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    Is there a reason why someone would want cat < /etc/fstab instead of just cat /etc/fstab? Or is cat just a suboptimal example here? Commented Jul 31, 2018 at 7:06
  • In the first case, cat opens the file, and in the second case, the shell opens the file, passing it as cat's standard input. quote from unix.stackexchange.com/questions/258931/… Commented Apr 19, 2019 at 0:53
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    I can't think of a reason why cat < file would ever be better than cat file. However, there are some cases where there is a difference -- for example, grep string file is different from grep string < file in that the second form doesn't prefix "file:" in front of every line. "cat file | grep string" is better written as "grep string < file". Not that any of this is worth doing for performance alone these days, but it is better coding practice. Commented Jul 9, 2019 at 3:43
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    @star In cat < /etc/fstab, the shell opens the file, passing it as cat's standard input, whereas in cat /etc/fstab, cat opens the file. I think you'd interchanged them. Commented Jun 5, 2021 at 13:11
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    @BrianC cat < file > foo would not create or overwrite foo if file could not be opened and so wouldn't potentially zap a file in an error scenario while cat file > foo would create/overwrite foo in that case. Commented Dec 26, 2021 at 14:09
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Take a look at the Bash man page. This notation is part of what's called a here documents & here strings. It allows you the ability to generate multi-line data input as one continuous string. The variation you're asking about is called a here string.

excerpt from Bash man page

Here Strings A variant of here documents, the format is: <<<word The word is expanded and supplied to the command on its standard input.

NOTE: For more info you can also check out the Bash Reference Manual which discusses Here Strings.

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It means here strings. 

<<< strings

The strings is expanded and supplied to the command on its standard input.

In your example, strings aaa is feed to sed command via stdin.

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