11. Variables - Part III

As we mentioned in Variables - Part I, curly brackets around a variable avoid confusion:

foo=sun
echo $fooshine     # $fooshine is undefined
echo ${foo}shine   # displays the word "sunshine"

That's not all, though - these fancy brackets have a another, much more powerful use. We can deal with issues of variables being undefined or null (in the shell, there's not much difference between undefined and null).

Using Default Values

Consider the following code snippet which prompts the user for input, but accepts defaults:

#!/bin/sh
echo -en "What is your name [ `whoami` ] "
read myname
if [ -z "$myname" ]; then
  myname=`whoami`
fi
echo "Your name is : $myname"

Passing the "-en" to echo tells it not to add a linebreak (for bash and csh). For Dash, Bourne and other compliant shells, you use a "\c" at the end of the line, instead. Ksh understands both forms.
This script runs like this if you accept the default by pressing "RETURN":

steve$ ./name.sh
What is your name [ steve ]
Your name is : steve

... or, with user input:

steve$ ./name.sh
What is your name [ steve ] foo
Your name is : foo

This could be done better using a shell variable feature. By using curly braces and the special ":-" usage, you can specify a default value to use if the variable is unset:

echo -en "What is your name [ `whoami` ] "
read myname
echo "Your name is : ${myname:-`whoami`}"

This could be considered a special case - we're using the output of the whoami command, which prints your login name (UID). The more canonical example is to use fixed text, like this:

echo "Your name is : ${myname:-John Doe}"

As with other use of the backticks, `whoami` runs in a subshell, so any cd commands, or setting any other variables, within the backticks, will not affect the currently-running shell.

Using and Setting Default Values

There is another syntax, ":=", which sets the variable to the default if it is undefined:

echo "Your name is : ${myname:=John Doe}"

This technique means that any subsequent access to the $myname variable will always get a value, either entered by the user, or "John Doe" otherwise.

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