Classic Shell Scripting

Common Extensions

Both bash and ksh93 support a large number of extensions over the POSIX shell. This section deals with those extensions that overlap; i.e., where both shells provide the same features, and in the same way.

The select Loop

bash and ksh share the select loop, which allows you to generate simple menus easily. It has concise syntax, but it does quite a lot of work. The syntax is:

select name [in list]
do
    statements that can use $name ...
done

This is the same syntax as the regular for loop except for the keyword select. And like for, you can omit the in list and it will default to "$@"; i.e., the list of quoted command-line arguments.

Here is what select does:

1. Generate a menu of each item in list, formatted with numbers for each choice

2. Print the value of PS3 as a prompt and waits for the user to enter a number

3. Store the selected choice in the variable name and the selected number in the built-in variable REPLY

4. Execute the statements in the body

5. Repeat the process forever (but see later for how to exit)

An example should help make this process clearer. Suppose you need to know how to set the TERM variable correctly for a timesharing system using different kinds of video display terminals. You don't have terminals hardwired to your computer; instead, your users communicate through a terminal server. Although the telnet protocol can pass the TERM environment variable, the terminal server isn't smart enough to do so. This means, among other things, that the tty (serial device) ...