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

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

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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marked as duplicate by don_crissti, Mat, Anthon, Michael Mrozek Jun 22 '13 at 20:47

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

4 Answers 4

up vote 10 down vote accepted

Others have answered the basic question: what is it?

Now, why is it 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 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. – glenn jackman Jun 22 '13 at 18:32
+1 for going the extra mile. – Joseph R. Jun 22 '13 at 18:37

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:


   The word is expanded and supplied to the command on its standard input.
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<<< denotes a here string.

$ cat <<< 'hi there'
hi there

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

<< denotes a here document.

$ cat <<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
bar bar
foo foo

< 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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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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