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Keep in mind that characters such as the dollar sign ($), back-tick (`), asterisk (*), exclamation point (!), and others have special meaning to the shell, which you will see as you proceed through this chapter. On some occasions, you want the shell to use these characters’ special meaning and other times you don't. For example, if you typed echo $HOME, the shell would think that you meant to display the name of your home directory (stored in the $HOME variable) to the screen (such as /home/chris) because a $ indicates a variable name follows that character.

If you wanted literally to show $HOME, you would need to escape the $. Typing echo '$HOME' or echo \$HOME would literally show $HOME on the screen. So, if you want to have the shell interpret a single character literally, precede it with a backslash (\). To have a whole set of characters interpreted literally, surround those characters with single quotes (').

Using double quotes is a bit trickier. Surround a set of text with double quotes if you want all but a few characters used literally. For example, with text surrounded with double quotes, dollar signs ($), back-ticks (`), and exclamation points (!) are interpreted specially, but other characters (such as an asterisk) are not. Type these three lines to see the different output (shown on the right):

  echo '$HOME * `date`' $HOME * `date` echo ″$HOME * `date`″ /home/chris * Tue Jan 21 16:56:52 EDT 2020 echo $HOME * `date` /home/chris file1 file2 Tue Jan 21 16:56:52 EDT 2020

Using variables is a great way to get information that can change from computer to computer or from day to day. The following example sets the output of the uname -n command to the MACHINE variable. Then I use parentheses to set NUM_FILES to the number of files in the current directory by piping (|) the output of the ls command to the word count command (wc -l):

 MACHINE=`uname -n` NUM_FILES=$(/bin/ls | wc -l)

Variables can also contain the value of other variables. This is useful when you have to preserve a value that will change so that you can use it later in the script. Here, BALANCE is set to the value of the CurBalance variable:

 BALANCE="$CurBalance"