10.4. Testing and Branching

Controlling program flow in a code block

case (in) / esac

The case construct is the shell equivalent of switch in C/C++. It permits branching to one of a number of code blocks, depending on condition tests. It serves as a kind of shorthand for multiple if/then/else statements and is an appropriate tool for creating menus.

case "$variable" in

 "$condition1" )
 command...
 ;;

 "$condition2" )
 command...
 ;;

esac

Quoting the variables is not mandatory, since word splitting does not take place. Each test line ends with a right paren ). Each condition block ends with a double semicolon ;;. The entire case block terminates with an esac (case spelled backwards).

Example 10-22. Using case

#!/bin/bash echo; echo "Hit a key, then hit return." read Keypress case "$Keypress" in [a-z] ) echo "Lowercase letter";; [A-Z] ) echo "Uppercase letter";; [0-9] ) echo "Digit";; * ) echo "Punctuation, whitespace, or other";; esac # Allows ranges of characters in [square brackets]. # Exercise for the reader: # As the script stands, # it accepts a single keystroke, then terminates. # Change the script so it accepts continuous input, # reports on each keystroke, and terminates only when "X" is hit. # Hint: enclose everything in a "while" loop. exit 0

Example 10-23. Creating menus using case

#!/bin/bash # Crude address database clear # Clear the screen. echo " Contact List" echo " ------- ----" echo "Choose one of the following persons:" echo echo "[E]vans, Roland" echo "[J]ones, Mildred" echo "[S]mith, Julie" echo "[Z]ane, Morris" echo read person case "$person" in # Note variable is quoted. "E" | "e" ) # Accept upper or lowercase input. echo echo "Roland Evans" echo "4321 Floppy Dr." echo "Hardscrabble, CO 80753" echo "(303) 734-9874" echo "(303) 734-9892 fax" echo "revans@zzy.net" echo "Business partner & old friend" ;; # Note double semicolon to terminate # each option. "J" | "j" ) echo echo "Mildred Jones" echo "249 E. 7th St., Apt. 19" echo "New York, NY 10009" echo "(212) 533-2814" echo "(212) 533-9972 fax" echo "milliej@loisaida.com" echo "Girlfriend" echo "Birthday: Feb. 11" ;; # Add info for Smith & Zane later. * ) # Default option. # Empty input (hitting RETURN) fits here, too. echo echo "Not yet in database." ;; esac echo # Exercise for the reader: # Change the script so it accepts continuous input, # instead of terminating after displaying just one address. exit 0

An exceptionally clever use of case involves testing for command-line parameters.

#! /bin/bash case "$1" in "") echo "Usage: ${0##*/} <filename>"; exit 65;; # No command-line parameters, # or first parameter empty. # Note that ${0##*/} is ${var##pattern} param substitution. Net result is $0. -*) FILENAME=./$1;; # If filename passed as argument ($1) starts with a dash, # replace it with ./$1 # so further commands don't interpret it as an option. * ) FILENAME=$1;; # Otherwise, $1. esac

Example 10-24. Using command substitution to generate the case variable

#!/bin/bash # Using command substitution to generate a "case" variable. case $( arch ) in # "arch" returns machine architecture. i386 ) echo "80386-based machine";; i486 ) echo "80486-based machine";; i586 ) echo "Pentium-based machine";; i686 ) echo "Pentium2+-based machine";; * ) echo "Other type of machine";; esac exit 0

A case construct can filter strings for globbing patterns.

Example 10-25. Simple string matching

#!/bin/bash # match-string.sh: simple string matching match_string () { MATCH=0 NOMATCH=90 PARAMS=2 # Function requires 2 arguments. BAD_PARAMS=91 [ $# -eq $PARAMS ] || return $BAD_PARAMS case "$1" in "$2") return $MATCH;; * ) return $NOMATCH;; esac } a=one b=two c=three d=two match_string $a # wrong number of parameters echo $? # 91 match_string $a $b # no match echo $? # 90 match_string $b $d # match echo $? # 0 exit 0

Example 10-26. Checking for alphabetic input

#!/bin/bash # Using "case" structure to filter a string. SUCCESS=0 FAILURE=-1 isalpha () # Tests whether *first character* of input string is alphabetic. { if [ -z "$1" ] # No argument passed? then return $FAILURE fi case "$1" in [a-zA-Z]*) return $SUCCESS;; # Begins with a letter? * ) return $FAILURE;; esac } # Compare this with "isalpha ()" function in C. isalpha2 () # Tests whether *entire string* is alphabetic. { [ $# -eq 1 ] || return $FAILURE case $1 in *[!a-zA-Z]*|"") return $FAILURE;; *) return $SUCCESS;; esac } check_var () # Front-end to isalpha(). { if isalpha "$@" then echo "$* = alpha" else echo "$* = non-alpha" # Also "non-alpha" if no argument passed. fi } a=23skidoo b=H3llo c=-What? d=`echo $b` # Command substitution. check_var $a check_var $b check_var $c check_var $d check_var # No argument passed, so what happens? # Script improved by S.C. exit 0

select

The select construct, adopted from the Korn Shell, is yet another tool for building menus.

select variable [in list]
do
 command...
 break
done

This prompts the user to enter one of the choices presented in the variable list. Note that select uses the PS3 prompt (#? ) by default, but that this may be changed.

Example 10-27. Creating menus using select

#!/bin/bash PS3='Choose your favorite vegetable: ' # Sets the prompt string. echo select vegetable in "beans" "carrots" "potatoes" "onions" "rutabagas" do echo echo "Your favorite veggie is $vegetable." echo "Yuck!" echo break # if no 'break' here, keeps looping forever. done exit 0

If in list is omitted, then select uses the list of command line arguments ($@) passed to the script or to the function in which the select construct is embedded.

Compare this to the behavior of a

for variable [in list]

construct with the in list omitted.

Example 10-28. Creating menus using select in a function

#!/bin/bash PS3='Choose your favorite vegetable: ' echo choice_of() { select vegetable # [in list] omitted, so 'select' uses arguments passed to function. do echo echo "Your favorite veggie is $vegetable." echo "Yuck!" echo break done } choice_of beans rice carrots radishes tomatoes spinach # $1 $2 $3 $4 $5 $6 # passed to choice_of() function exit 0