## 5.4. select

Almost all of the flow-control constructs we have seen so far are also available in the Bourne shell, and the C shell has equivalents with different syntax. Our next construct, `select`, is unique to the Korn shell; moreover, it has no analogue in conventional programming languages.

`select` 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 `in` `list`, and it will default to `"$@"`, i.e., the list of quoted command-line arguments.

Here is what `select` does:

- Generates a menu of each item in list, formatted with numbers for each choice

- Prompts the user for a number (with the value of `PS3`)

- Stores the selected choice in the variable name and the selected number in the built-in variable `REPLY`

- Executes the statements in the body

- Repeats the process forever (but see below for how to exit)

Once again, an example should help make this process clearer. Assume you need to write the code for Task 5-4, but your life is not as simple. You don't have terminals hardwired to your computer; instead, your users communicate through a terminal server, or they log in remotely, via telnet or ssh. This means, among other things, that the tty number does not determine the type of terminal.

Therefore, you have no choice but to prompt the user for his or her terminal type at login time. To do this, you can put the following code in /etc/profile (assume you have the same choice of terminal types):

>     PS3='terminal? '
>     select term in gl35a t2000 s531 vt99; do
>         if [[ -n $term ]]; then
>             TERM=$term
>             print TERM is $TERM
>             export TERM
>             break
>         else
>             print 'invalid.'
>         fi
>     done

When you run this code, you see this menu:

>     1) gl35a
>     2) t2000
>     3) s531
>     4) vt99
>     terminal?

The built-in shell variable `PS3` contains the prompt string that `select` uses; its default value is the not particularly useful "`#?` ". So the first line of the above code sets it to a more relevant value.[79]

> [79] As for `PS1`, ksh does parameter, command, and arithmetic substitution on the value before printing it.

The `select` statement constructs the menu from the list of choices. If the user enters a valid number (from 1 to 4), the variable `term` is set to the corresponding value; otherwise it is null. (If the user just presses ENTER, the shell prints the menu again.)

The code in the loop body checks if `term` is non-null. If so, it assigns `$term` to the environment variable `TERM`, exports `TERM`, and prints a confirmation message; then the break statement exits the `select` loop. If `term` is null, the code prints an error message and repeats the prompt (but not the menu).

The break statement is the usual way of exiting a `select` loop. Actually (like its analogue in C), it can be used to exit any surrounding control structure we've seen so far (except `case`, where the double-semicolons act like break) as well as the `while` and `until` we will see soon. We haven't introduced break until now because some people consider it to be bad coding style to use it to exit a loop. However, it is necessary for exiting `select` when the user makes a valid choice. (A user can also type CTRL-D -- for end-of-input -- to get out of a `select` loop. This gives the user a uniform way of exiting, but it doesn't help the shell programmer much.)

Let's refine our solution by making the menu more user-friendly, so that the user doesn't have to know the terminfo name of his or her terminal. We do this by using quoted character strings as menu items and then using `case` to determine the terminfo name:

>     print 'Select your terminal type:'
>     PS3='terminal? '
>     select term in \
>         'Givalt GL35a' \
>         'Tsoris T-2000' \
>         'Shande 531' \
>         'Vey VT99'
>     do
>         case $REPLY in
>             1 ) TERM=gl35a ;;
>             2 ) TERM=t2000 ;;
>             3 ) TERM=s531 ;;
>             4 ) TERM=vt99 ;;
>             * ) print 'invalid.' ;;
>         esac
>         if [[ -n $term ]]; then
>             print TERM is $TERM
>             export TERM
>             break
>         fi
>     done

This code looks a bit more like a menu routine in a conventional program, though `select` still provides the shortcut of converting the menu choices into numbers. We list each of the menu choices on its own line for reasons of readability, but once again we need continuation characters to keep the shell from complaining about syntax.

Here is what the user sees when this code is run:

>     Select your terminal type:
>     1) Givalt GL35a
>     2) Tsoris T-2000
>     3) Shande 531
>     4) Vey VT99
>     terminal?

This is a bit more informative than the previous code's output.

When the body of the `select` loop is entered, `$term` equals one of the four strings (or is null if the user made an invalid choice), while the built-in variable `REPLY` contains the number the user selected. We need a `case` statement to assign the correct value to `TERM`; we use the value of `REPLY` as the `case` selector.

Once the `case` statement is finished, the `if` checks to see if a valid choice was made, as in the previous solution. If the choice was valid, `TERM` has already been assigned, so the code just prints a confirmation message, exports `TERM`, and exits the `select` loop. If it wasn't valid, the `select` loop repeats the prompt and goes through the process again.

Within a `select` loop, if `REPLY` is set to the null string, the shell reprints the menu. This happens, as mentioned, when the user hits ENTER. But you may also explicitly set `REPLY` to the null string to force the shell to reprint the menu.

The variable `TMOUT` (time out) can affect the `select` statement. Before the `select` loop, set it to some number of seconds N, and if nothing is entered within that amount of time, the `select` will exit. As will be explained later, `TMOUT` also affects the read command and the shell's interactive prompting mechanism.

Copyright © 2003 O'Reilly & Associates. All rights reserved.