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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 for except for the keyword select. And like for, you can omit the in list and it will default to "[email protected]", i.e., the list of quoted command-line arguments.
Here is what select does:
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. 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 break else print 'invalid.' fi done
If you run this code, you will 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
The select statement constructs the menu from the list of choices. If the user enters a valid number (from 1 to 4), then the variable term is set to the corresponding value; otherwise it is null. (If the user just presses RETURN, 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 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 analog 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 it is considered 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 termcap name:
print 'Select your terminal type:' PS3='terminal? ' select term in \ 'Givalt VT100' \ 'Tsoris VT220' \ 'Shande VT320' \ 'Vey VT520' 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 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 will see 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 selects. 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, then TERM has already been assigned, so the code just prints a confirmation message and exits the select loop. If it wasn't valid, the select loop repeats the prompt and goes through the process again.
The select statement is used to generate a menu list if you are writing a shell program that expects input from the user online. The format of the select statement is as follows:
select item in itemlist do Statements done
itemlist is optional. If it's not provided, the system iterates through the entries in item one at a time. If itemlist is provided, however, the system iterates for each entry in itemlist and the current value of itemlist is assigned to item for each iteration, which then can be used as part of the statements being executed.
If you want to write a menu that gives the user a choice of picking a Continue or a Finish, you can write the following shell program:
#!/bin/bash select item in Continue Finish do if [ $item = "Finish" ]; then break fi done
When the select command is executed, the system displays a menu with numeric choices to the user--in this case, 1 for Continue, and 2 for Finish. If the user chooses 1, the variable item contains a value of Continue; if the user chooses 2, the variable item contains a value of Finish. When the user chooses 2, the if statement is executed and the loop terminates.
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