Linux Pocket Guide, 2nd Edition

Variables

We described shell variables earlier:

$ MYVAR=6
$ echo $MYVAR
6

All values held in variables are strings, but if they are numeric, the shell will treat them as numbers when appropriate.

$ NUMBER="10"
$ expr $NUMBER + 5
15

When you refer to a variable’s value in a shell script, it’s a good idea to surround it with double quotes to prevent certain runtime errors. An undefined variable, or a variable with spaces in its value, will evaluate to something unexpected if not surrounded by quotes, causing your script to malfunction.

$ FILENAME="My Document"            Space in the name
$ ls $FILENAME                      Try to list it
ls: My: No such file or directory   Oops! ls saw 2 arguments
ls: Document: No such file or directory
$ ls -l "$FILENAME"                 List it properly
My Document                         ls saw only 1 argument

If a variable name is evaluated adjacent to another string, surround it with curly braces to prevent unexpected behavior:

$ HAT="fedora"
$ echo "The plural of $HAT is $HATs"
The plural of fedora is             Oops! No variable "HATs"
$ echo  "The plural of $HAT is ${HAT}s"
The plural of fedora is fedoras     What we wanted