What does <<< mean?

Question

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?

it seems `<` is for passing file (or directory), `<< @` for passing multiple lines (similar to the `banner` command in [tag: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. — , Oct 05 '17 at 10:32

score 288 · Accepted Answer · edited Jul 29 '23 at 15:04

288

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!

edited Jul 29 '23 at 15:04

Matthias Braun

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answered Jun 22 '13 at 18:31

glenn jackman

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In addition to here-strings, [process substutitions](http://www.gnu.org/software/bash/manual/bashref.html#Process-Substitution) are very useful for the same reasons. – glenn jackman Jun 22 '13 at 18:32
11

This is a fantastic explanation of <<< as well as pipes and sub-shells! I learned both from this. – David Mann May 12 '17 at 01:29
3

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`) – glenn jackman Aug 09 '17 at 20:00
@glennjackman, What's the different between here-strings vs replacing it with an echoed temporary file? – Pacerier Mar 06 '18 at 10:59
Functionally nothing. With a here string, you don't have to delete a temp file. – glenn jackman Mar 06 '18 at 11:03
Doesn't seem to work on Windows' Ubuntu Bash: `dradcliffe:~$ read first second <<< big joe // nothing` – Dean Radcliffe May 23 '18 at 21:07
1

First the here string is a single string so if it contains whitespace you need quotes. Second the read command doesn't output anything so you'll have to clarify "nothing" – glenn jackman May 23 '18 at 21:43
3

Very interesting. The case you give does not result in unexpected behavior using zsh. – Ryan Ward May 08 '19 at 21:00
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On my zsh, the output for `echo $second $first` is `world hello` – aafulei Jul 25 '19 at 04:28
For zsh the last command of a pipeline is run in the parent shell ([blog post](https://www.vidarholen.net/contents/blog/?p=178)), though unfortunately I couldn't(?) infer this directly from the [docs](http://zsh.sourceforge.net/Doc/Release/Shell-Grammar.html#Simple-Commands-_0026-Pipelines). Anyway, when to use a subshell is apparently [not specified by posix](https://unix.stackexchange.com/questions/80362/what-does-mean). – Ben Jul 24 '20 at 13:04
Am I right to think, this is a "bashism"? It's not backed by POSIX, is it? – d.c. Nov 21 '22 at 20:09
bash usually steals features from Korn shell or zsh. I don't know who invented it. – glenn jackman Nov 22 '22 at 00:47

score 146 · Answer 2 · edited Oct 05 '17 at 10:23

146

<<< 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
 ...

edited Oct 05 '17 at 10:23

RomanPerekhrest

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answered Jun 22 '13 at 17:40

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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? – Gerrit-K Jul 31 '18 at 07:06
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 https://unix.stackexchange.com/questions/258931/difference-between-cat-and-cat – star Apr 19 '19 at 00:53
7

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. – Brian C Jul 09 '19 at 03:43
@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. – Ray Jasson Jun 05 '21 at 13:11
@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. – Ed Morton Dec 26 '21 at 14:09
Hi @ed-morton - In testing, `cat < foo > file` doesn't actually overwrite foo; and If there is an rare error reading the input file, `cat foo` is able to report the file name, whereas it cannot when presented with a stream via `< file`. Of course, BOTH forms are absolutely terrible shell programming practice and should never be done. Instead, use the `cp` command, or output to an intermediate file and use `if [ -s foo.tmp ]` to test for output file validity before then moving the output over the top of any existing file called foo. – Brian C Dec 27 '21 at 09:37
@BrianC I didn't suggest that `cat < foo > file` would overwrite `foo`, of course it doesn't. You misread my comment, it has nothing to do with overwriting the input file. I was providing a case where `cat < file` would be better than `cat file` in response to your comment ["I can't think of a reason why cat < file would ever be better than cat file"](https://unix.stackexchange.com/questions/80362/what-does-mean/80368?noredirect=1#comment978966_80368) – Ed Morton Dec 27 '21 at 12:18

slm · Answer 3 · 2023-08-01T16:54:59.600

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.

score 9 · Answer 4 · answered Jun 22 '13 at 16:42

9

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.

answered Jun 22 '13 at 16:42

cuonglm

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What does <<< mean?

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