11.4. Testing and Branching

Controlling program flow in a code block

case (in) / esac

The case construct is the shell scripting analog to 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 ;;. If a condition tests true, then the associated commands execute and the case block terminates. The entire case block ends with an esac (case spelled backwards).

Example 11-24. Using case

#!/bin/bash # Testing ranges of characters. echo; echo "Hit a key, then hit return." read Keypress case "$Keypress" in [[:lower:]] ) echo "Lowercase letter";; [[:upper:]] ) echo "Uppercase letter";; [0-9] ) echo "Digit";; * ) echo "Punctuation, whitespace, or other";; esac # Allows ranges of characters in [square brackets], #+ or POSIX ranges in [[double square brackets. # In the first version of this example, #+ the tests for lowercase and uppercase characters were #+ [a-z] and [A-Z]. # This no longer works in certain locales and/or Linux distros. # POSIX is more portable. # Thanks to Frank Wang for pointing this out. # Exercise: # -------- # As the script stands, it accepts a single keystroke, then terminates. # Change the script so it accepts repeated input, #+ reports on each keystroke, and terminates only when "X" is hit. # Hint: enclose everything in a "while" loop. exit 0

Example 11-25. 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 Flash 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 "Ex-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: # -------- # Change the script so it accepts multiple inputs, #+ 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 $E_PARAM;; # 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

Here is an more straightforward example of command-line parameter handling:

#! /bin/bash while [ $# -gt 0 ]; do # Until you run out of parameters . . . case "$1" in -d|--debug) # "-d" or "--debug" parameter? DEBUG=1 ;; -c|--conf) CONFFILE="$2" shift if [ ! -f $CONFFILE ]; then echo "Error: Supplied file doesn't exist!" exit $E_CONFFILE # File not found error. fi ;; esac shift # Check next set of parameters. done # From Stefano Falsetto's "Log2Rot" script, #+ part of his "rottlog" package. # Used with permission.

Example 11-26. Using command substitution to generate the case variable

#!/bin/bash # case-cmd.sh: Using command substitution to generate a "case" variable. case $( arch ) in # "arch" returns machine architecture. # Equivalent to 'uname -m' ... 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 11-27. Simple string matching

#!/bin/bash # match-string.sh: Simple string matching. match_string () { # Exact string match. MATCH=0 E_NOMATCH=90 PARAMS=2 # Function requires 2 arguments. E_BAD_PARAMS=91 [ $# -eq $PARAMS ] || return $E_BAD_PARAMS case "$1" in "$2") return $MATCH;; * ) return $E_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 11-28. Checking for alphabetic input

#!/bin/bash # isalpha.sh: Using a "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 } isdigit () # Tests whether *entire string* is numerical. { # In other words, tests for integer variable. [ $# -eq 1 ] || return $FAILURE case $1 in *[!0-9]*|"") return $FAILURE;; *) return $SUCCESS;; esac } check_var () # Front-end to isalpha (). { if isalpha "$@" then echo "\"$*\" begins with an alpha character." if isalpha2 "$@" then # No point in testing if first char is non-alpha. echo "\"$*\" contains only alpha characters." else echo "\"$*\" contains at least one non-alpha character." fi else echo "\"$*\" begins with a non-alpha character." # Also "non-alpha" if no argument passed. fi echo } digit_check () # Front-end to isdigit (). { if isdigit "$@" then echo "\"$*\" contains only digits [0 - 9]." else echo "\"$*\" has at least one non-digit character." fi echo } a=23skidoo b=H3llo c=-What? d=What? e=`echo $b` # Command substitution. f=AbcDef g=27234 h=27a34 i=27.34 check_var $a check_var $b check_var $c check_var $d check_var $e check_var $f check_var # No argument passed, so what happens? # digit_check $g digit_check $h digit_check $i exit 0 # Script improved by S.C. # Exercise: # -------- # Write an 'isfloat ()' function that tests for floating point numbers. # Hint: The function duplicates 'isdigit ()', #+ but adds a test for a mandatory decimal point.

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 this may be changed.

Example 11-29. Creating menus using select

#!/bin/bash PS3='Choose your favorite vegetable: ' # Sets the prompt string. # Otherwise it defaults to #? . echo select vegetable in "beans" "carrots" "potatoes" "onions" "rutabagas" do echo echo "Your favorite veggie is $vegetable." echo "Yuck!" echo break # What happens if there is no 'break' here? done exit # Exercise: # -------- # Fix this script to accept user input not specified in #+ the "select" statement. # For example, if the user inputs "peas," #+ the script would respond "Sorry. That is not on the menu."

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

Compare this to the behavior of a

for variable [in list]

construct with the in list omitted.

Example 11-30. 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

See also Example 36-3.