O'Reilly logo

Shell Scripting: Expert Recipes for Linux, Bash, and More by Steve Parker

Stay ahead with the world's most comprehensive technology and business learning platform.

With Safari, you learn the way you learn best. Get unlimited access to videos, live online training, learning paths, books, tutorials, and more.

Start Free Trial

No credit card required

Using Variables

When assigning a variable, the name is not preceded by a dollar sign: variable=value. When referencing a variable, it is preceded by a dollar sign: echo $variable. Actually, the $variable syntax is a special case, but it is sufficient most of the time. Variables are properly referenced as ${variable}, as this allows the shell to differentiate between ${var}iable (the variable $var followed by the text “iable”) and ${variable} (the variable $variable). This can be useful when applying suffixes to variables, such as “${kb}Kb is $bytes bytes, or approx ${mb}Mb”:

cat mb2.sh
echo -n "Enter a size in Kb: "
read kb
bytes='expr $kb \* 1024'
mb='expr $kb / 1024'
echo "${kb}Kb is ${bytes} bytes, or approx ${mb}Mb."
$ ./mb2.sh
Enter a size in Kb: 12345
12345Kb is 12641280 bytes, or approx 12Mb.
$

If the curly brackets were not there, then ${kb}Kb would become $kbKb, and as a variable called kbKb has not been defined, it will evaluate to the empty string.

note.ai

In the context of a string, an undefined variable is interpreted as an empty string. If it were being treated as a number, it would be interpreted as zero.

With the curly brackets removed, the script runs like this:

cat mb1.sh
echo -n "Enter a size in Kb: "
read kb
bytes='expr $kb \* 1024'
mb='expr $kb / 1024'
echo "$kbKb is $bytes bytes, or approx $mbMb."
$ ./mb1.sh Enter a size in Kb: 12345   is 12641280 bytes, or ...

With Safari, you learn the way you learn best. Get unlimited access to videos, live online training, learning paths, books, interactive tutorials, and more.

Start Free Trial

No credit card required