Assigning variables default values

Line 168 shows an example of providing a default value for a variable. We have already seen how to assign a value to a variable. For example:

value=$newvalue

This assigns the value of newvalue to $value. But there are times when we want to provide a default option, in case $newvalue is bogus (for example, if the user accidentally pressed <Enter> instead of entering a name). An assignment of the form variable=${value:-default} assigns value to $variable if it is set: otherwise it assigns default to $variable. In the example above, the variable ${PAGER:-more} is expanded to either the value of $PAGER, or if this is not set, to more.

For example, here is get_fname:

   94 : get_fname ()
   95 : {
   96 :   echo "Enter a filename: \c"
   97 :   read newfname
   98 :   fname=${newfname:-${fname}}
   99 : }

There are other uses for this mechanism. For example:

   117 :   newdir=${newdir:-`pwd`}

Variations exist on the default behavior for a variable assignment. Some of the most common variable substitutions you can use are as follows:

${var:-word}

If var is set and not empty, substitute the value of var; otherwise substitute word.

${var:=word}

If var is not set or is empty, set it to word; then substitute the value of var (that is, if $var does not exist, set $var to $word and use that).

${var:?word}

If var is set and not empty, substitute the value of var; otherwise print word and exit from the shell.

${var:+word}

If var is set and not empty, substitute the value of word; otherwise substitute nothing.