Softpanorama
May the source be with you, but remember the KISS principle ;-)

Contents Bulletin Scripting in shell and Perl Network troubleshooting History Humor

Bash select statement

News

bash

Recommended  Links

Education Bash Control Structures Advanced navigation
if statements Loops in Shell case select Sequences Pipes in Loops
Arithmetic expressions Comparison operators BASH Debugging Shell history Humor  Etc

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 "$@", 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 value.

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 (pdksh)

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.

Recommended Links

Softpanorama hot topic of the month

Softpanorama Recommended

InformIT Red Hat Linux 7 Unleashed / Iteration Statements



Etc

FAIR USE NOTICE This site contains copyrighted material the use of which has not always been specifically authorized by the copyright owner. We are making such material available in our efforts to advance understanding of environmental, political, human rights, economic, democracy, scientific, and social justice issues, etc. We believe this constitutes a 'fair use' of any such copyrighted material as provided for in section 107 of the US Copyright Law. In accordance with Title 17 U.S.C. Section 107, the material on this site is distributed without profit exclusivly for research and educational purposes.   If you wish to use copyrighted material from this site for purposes of your own that go beyond 'fair use', you must obtain permission from the copyright owner. 

ABUSE: IPs or network segments from which we detect a stream of probes might be blocked for no less then 90 days. Multiple types of probes increase this period.  

Society

Groupthink : Two Party System as Polyarchy : Corruption of Regulators : Bureaucracies : Understanding Micromanagers and Control Freaks : Toxic Managers :   Harvard Mafia : Diplomatic Communication : Surviving a Bad Performance Review : Insufficient Retirement Funds as Immanent Problem of Neoliberal Regime : PseudoScience : Who Rules America : Neoliberalism  : The Iron Law of Oligarchy : Libertarian Philosophy

Quotes

War and Peace : Skeptical Finance : John Kenneth Galbraith :Talleyrand : Oscar Wilde : Otto Von Bismarck : Keynes : George Carlin : Skeptics : Propaganda  : SE quotes : Language Design and Programming Quotes : Random IT-related quotesSomerset Maugham : Marcus Aurelius : Kurt Vonnegut : Eric Hoffer : Winston Churchill : Napoleon Bonaparte : Ambrose BierceBernard Shaw : Mark Twain Quotes

Bulletin:

Vol 25, No.12 (December, 2013) Rational Fools vs. Efficient Crooks The efficient markets hypothesis : Political Skeptic Bulletin, 2013 : Unemployment Bulletin, 2010 :  Vol 23, No.10 (October, 2011) An observation about corporate security departments : Slightly Skeptical Euromaydan Chronicles, June 2014 : Greenspan legacy bulletin, 2008 : Vol 25, No.10 (October, 2013) Cryptolocker Trojan (Win32/Crilock.A) : Vol 25, No.08 (August, 2013) Cloud providers as intelligence collection hubs : Financial Humor Bulletin, 2010 : Inequality Bulletin, 2009 : Financial Humor Bulletin, 2008 : Copyleft Problems Bulletin, 2004 : Financial Humor Bulletin, 2011 : Energy Bulletin, 2010 : Malware Protection Bulletin, 2010 : Vol 26, No.1 (January, 2013) Object-Oriented Cult : Political Skeptic Bulletin, 2011 : Vol 23, No.11 (November, 2011) Softpanorama classification of sysadmin horror stories : Vol 25, No.05 (May, 2013) Corporate bullshit as a communication method  : Vol 25, No.06 (June, 2013) A Note on the Relationship of Brooks Law and Conway Law

History:

Fifty glorious years (1950-2000): the triumph of the US computer engineering : Donald Knuth : TAoCP and its Influence of Computer Science : Richard Stallman : Linus Torvalds  : Larry Wall  : John K. Ousterhout : CTSS : Multix OS Unix History : Unix shell history : VI editor : History of pipes concept : Solaris : MS DOSProgramming Languages History : PL/1 : Simula 67 : C : History of GCC developmentScripting Languages : Perl history   : OS History : Mail : DNS : SSH : CPU Instruction Sets : SPARC systems 1987-2006 : Norton Commander : Norton Utilities : Norton Ghost : Frontpage history : Malware Defense History : GNU Screen : OSS early history

Classic books:

The Peter Principle : Parkinson Law : 1984 : The Mythical Man-MonthHow to Solve It by George Polya : The Art of Computer Programming : The Elements of Programming Style : The Unix Haterís Handbook : The Jargon file : The True Believer : Programming Pearls : The Good Soldier Svejk : The Power Elite

Most popular humor pages:

Manifest of the Softpanorama IT Slacker Society : Ten Commandments of the IT Slackers Society : Computer Humor Collection : BSD Logo Story : The Cuckoo's Egg : IT Slang : C++ Humor : ARE YOU A BBS ADDICT? : The Perl Purity Test : Object oriented programmers of all nations : Financial Humor : Financial Humor Bulletin, 2008 : Financial Humor Bulletin, 2010 : The Most Comprehensive Collection of Editor-related Humor : Programming Language Humor : Goldman Sachs related humor : Greenspan humor : C Humor : Scripting Humor : Real Programmers Humor : Web Humor : GPL-related Humor : OFM Humor : Politically Incorrect Humor : IDS Humor : "Linux Sucks" Humor : Russian Musical Humor : Best Russian Programmer Humor : Microsoft plans to buy Catholic Church : Richard Stallman Related Humor : Admin Humor : Perl-related Humor : Linus Torvalds Related humor : PseudoScience Related Humor : Networking Humor : Shell Humor : Financial Humor Bulletin, 2011 : Financial Humor Bulletin, 2012 : Financial Humor Bulletin, 2013 : Java Humor : Software Engineering Humor : Sun Solaris Related Humor : Education Humor : IBM Humor : Assembler-related Humor : VIM Humor : Computer Viruses Humor : Bright tomorrow is rescheduled to a day after tomorrow : Classic Computer Humor

The Last but not Least


Copyright © 1996-2016 by Dr. Nikolai Bezroukov. www.softpanorama.org was created as a service to the UN Sustainable Development Networking Programme (SDNP) in the author free time. This document is an industrial compilation designed and created exclusively for educational use and is distributed under the Softpanorama Content License.

The site uses AdSense so you need to be aware of Google privacy policy. You you do not want to be tracked by Google please disable Javascript for this site. This site is perfectly usable without Javascript.

Original materials copyright belong to respective owners. Quotes are made for educational purposes only in compliance with the fair use doctrine.

FAIR USE NOTICE This site contains copyrighted material the use of which has not always been specifically authorized by the copyright owner. We are making such material available to advance understanding of computer science, IT technology, economic, scientific, and social issues. We believe this constitutes a 'fair use' of any such copyrighted material as provided by section 107 of the US Copyright Law according to which such material can be distributed without profit exclusively for research and educational purposes.

This is a Spartan WHYFF (We Help You For Free) site written by people for whom English is not a native language. Grammar and spelling errors should be expected. The site contain some broken links as it develops like a living tree...

You can use PayPal to make a contribution, supporting development of this site and speed up access. In case softpanorama.org is down you can use the at softpanorama.info

Disclaimer:

The statements, views and opinions presented on this web page are those of the author (or referenced source) and are not endorsed by, nor do they necessarily reflect, the opinions of the author present and former employers, SDNP or any other organization the author may be associated with. We do not warrant the correctness of the information provided or its fitness for any purpose.

Last modified: July 13, 2017