One of the most useful features of shell scripts is the lowly back quote character, usually called the backtick (`) in the Linux world. Be careful—this is not the normal single quotation mark character you are used to using for strings. Because it is not used very often outside of shell scripts, you may not even know where to find it on your keyboard. You should become familiar with it, because it’s a crucial component of many shell scripts.
Hint: On a U.S. keyboard, it is usually on the same key as the tilde symbol ( ∼ ).
The backtick allows you to assign the output of a shell command to a variable. While this doesn’t seem like much, it is a major building block in script programming. …..
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Backtick (`) symbol in Linux Shell Scripting (NextStep4it)