Ubuntu Unleashed 2013 Edition: Covering 12.10 and 13.04, Eighth Edition

Using the Backtick to Replace a String with Output

You can use the backtick (`) character to signal the shell to replace a string with its output when executed. This is called command substitution. You can use this special character in shell programs when you want the result of the execution of a command to be stored in a variable. For example, if you want to count the number of lines in a file called test.txt in the current directory and store the result in a variable called var, you can use the following command: