ヒアストリング (<<<)
参考：
bash で複数行の文字列の代入と、ヒアドキュメントを使わずに変数に格納する方法 – それマグで！
Bash Scripting Here Strings examples | LandofLinux.com
Here strings – Linux Shell Scripting Tutorial – A Beginner’s handbook
Here Strings – Advanced Bash-Scripting Guide | The Linux Documentation Project
How to pass the value of a variable to the stdin of a command? – Stack Overflow
What does <<< mean? – Unix & Linux Stack Exchange
What’s the difference between <<, <<< and < < in bash? – Ask Ubuntu