10.4. Testing and Branching
The case and select constructs are technically not loops, since they do not iterate the execution of a code block. Like loops, however, they direct program flow according to conditions at the top or bottom of the block.
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...
;;
"$condition2" )
command...
;;
esac
Quoting the variables is not mandatory, since word splitting does not take place.
Each test line ends with a right paren ).
Each condition block ends with a double semicolon ;;.
The entire case block terminates with an esac (case spelled backwards).
Example 10-24. Using case
1 #!/bin/bash 2 3 echo; echo "Hit a key, then hit return." 4 read Keypress 5 6 case "$Keypress" in 7 [a-z] ) echo "Lowercase letter";; 8 [A-Z] ) echo "Uppercase letter";; 9 [0-9] ) echo "Digit";; 10 * ) echo "Punctuation, whitespace, or other";; 11 esac # Allows ranges of characters in [square brackets]. 12 13 # Exercise: 14 # -------- 15 # As the script stands, # it accepts a single keystroke, then terminates. 16 # Change the script so it accepts continuous input, 17 # reports on each keystroke, and terminates only when "X" is hit. 18 # Hint: enclose everything in a "while" loop. 19 20 exit 0
Example 10-25. Creating menus using case
1 #!/bin/bash 2 3 # Crude address database 4 5 clear # Clear the screen. 6 7 echo " Contact List" 8 echo " ------- ----" 9 echo "Choose one of the following persons:" 10 echo 11 echo "[E]vans, Roland" 12 echo "[J]ones, Mildred" 13 echo "[S]mith, Julie" 14 echo "[Z]ane, Morris" 15 echo 16 17 read person 18 19 case "$person" in 20 # Note variable is quoted. 21 22 "E" | "e" ) 23 # Accept upper or lowercase input. 24 echo 25 echo "Roland Evans" 26 echo "4321 Floppy Dr." 27 echo "Hardscrabble, CO 80753" 28 echo "(303) 734-9874" 29 echo "(303) 734-9892 fax" 30 echo "revans@zzy.net" 31 echo "Business partner & old friend" 32 ;; 33 # Note double semicolon to terminate each option. 34 35 "J" | "j" ) 36 echo 37 echo "Mildred Jones" 38 echo "249 E. 7th St., Apt. 19" 39 echo "New York, NY 10009" 40 echo "(212) 533-2814" 41 echo "(212) 533-9972 fax" 42 echo "milliej@loisaida.com" 43 echo "Girlfriend" 44 echo "Birthday: Feb. 11" 45 ;; 46 47 # Add info for Smith & Zane later. 48 49 * ) 50 # Default option. 51 # Empty input (hitting RETURN) fits here, too. 52 echo 53 echo "Not yet in database." 54 ;; 55 56 esac 57 58 echo 59 60 # Exercise: 61 # -------- 62 # Change the script so it accepts continuous input, 63 #+ instead of terminating after displaying just one address. 64 65 exit 0
An exceptionally clever use of case involves testing for command-line parameters.
1 #! /bin/bash 2 3 case "$1" in 4 "") echo "Usage: ${0##*/} <filename>"; exit 65;; # No command-line parameters, 5 # or first parameter empty. 6 # Note that ${0##*/} is ${var##pattern} param substitution. Net result is $0. 7 8 -*) FILENAME=./$1;; # If filename passed as argument ($1) starts with a dash, 9 # replace it with ./$1 10 # so further commands don't interpret it as an option. 11 12 * ) FILENAME=$1;; # Otherwise, $1. 13 esacExample 10-26. Using command substitution to generate the case variable
1 #!/bin/bash 2 # Using command substitution to generate a "case" variable. 3 4 case $( arch ) in # "arch" returns machine architecture. 5 i386 ) echo "80386-based machine";; 6 i486 ) echo "80486-based machine";; 7 i586 ) echo "Pentium-based machine";; 8 i686 ) echo "Pentium2+-based machine";; 9 * ) echo "Other type of machine";; 10 esac 11 12 exit 0
A case construct can filter strings for globbing patterns.
Example 10-27. Simple string matching
1 #!/bin/bash 2 # match-string.sh: simple string matching 3 4 match_string () 5 { 6 MATCH=0 7 NOMATCH=90 8 PARAMS=2 # Function requires 2 arguments. 9 BAD_PARAMS=91 10 11 [ $# -eq $PARAMS ] || return $BAD_PARAMS 12 13 case "$1" in 14 "$2") return $MATCH;; 15 * ) return $NOMATCH;; 16 esac 17 18 } 19 20 21 a=one 22 b=two 23 c=three 24 d=two 25 26 27 match_string $a # wrong number of parameters 28 echo $? # 91 29 30 match_string $a $b # no match 31 echo $? # 90 32 33 match_string $b $d # match 34 echo $? # 0 35 36 37 exit 0Example 10-28. Checking for alphabetic input
1 #!/bin/bash 2 # isalpha.sh: Using a "case" structure to filter a string. 3 4 SUCCESS=0 5 FAILURE=-1 6 7 isalpha () # Tests whether *first character* of input string is alphabetic. 8 { 9 if [ -z "$1" ] # No argument passed? 10 then 11 return $FAILURE 12 fi 13 14 case "$1" in 15 [a-zA-Z]*) return $SUCCESS;; # Begins with a letter? 16 * ) return $FAILURE;; 17 esac 18 } # Compare this with "isalpha ()" function in C. 19 20 21 isalpha2 () # Tests whether *entire string* is alphabetic. 22 { 23 [ $# -eq 1 ] || return $FAILURE 24 25 case $1 in 26 *[!a-zA-Z]*|"") return $FAILURE;; 27 *) return $SUCCESS;; 28 esac 29 } 30 31 isdigit () # Tests whether *entire string* is numerical. 32 { # In other words, tests for integer variable. 33 [ $# -eq 1 ] || return $FAILURE 34 35 case $1 in 36 *[!0-9]*|"") return $FAILURE;; 37 *) return $SUCCESS;; 38 esac 39 } 40 41 42 43 check_var () # Front-end to isalpha (). 44 { 45 if isalpha "$@" 46 then 47 echo "\"$*\" begins with an alpha character." 48 if isalpha2 "$@" 49 then # No point in testing if first char is non-alpha. 50 echo "\"$*\" contains only alpha characters." 51 else 52 echo "\"$*\" contains at least one non-alpha character." 53 fi 54 else 55 echo "\"$*\" begins with a non-alpha character." 56 # Also "non-alpha" if no argument passed. 57 fi 58 59 echo 60 61 } 62 63 digit_check () # Front-end to isdigit (). 64 { 65 if isdigit "$@" 66 then 67 echo "\"$*\" contains only digits [0 - 9]." 68 else 69 echo "\"$*\" has at least one non-digit character." 70 fi 71 72 echo 73 74 } 75 76 a=23skidoo 77 b=H3llo 78 c=-What? 79 d=What? 80 e=`echo $b` # Command substitution. 81 f=AbcDef 82 g=27234 83 h=27a34 84 i=27.34 85 86 check_var $a 87 check_var $b 88 check_var $c 89 check_var $d 90 check_var $e 91 check_var $f 92 check_var # No argument passed, so what happens? 93 # 94 digit_check $g 95 digit_check $h 96 digit_check $i 97 98 99 exit 0 # Script improved by S.C. 100 101 # Exercise: 102 # -------- 103 # Write an 'isfloat ()' function that tests for floating point numbers. 104 # Hint: The function duplicates 'isdigit ()', 105 #+ 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
doneThis 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 10-29. Creating menus using select
1 #!/bin/bash 2 3 PS3='Choose your favorite vegetable: ' # Sets the prompt string. 4 5 echo 6 7 select vegetable in "beans" "carrots" "potatoes" "onions" "rutabagas" 8 do 9 echo 10 echo "Your favorite veggie is $vegetable." 11 echo "Yuck!" 12 echo 13 break # if no 'break' here, keeps looping forever. 14 done 15 16 exit 0
If in list is omitted, then select uses the list of command line arguments ($@) 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 10-30. Creating menus using select in a function
1 #!/bin/bash 2 3 PS3='Choose your favorite vegetable: ' 4 5 echo 6 7 choice_of() 8 { 9 select vegetable 10 # [in list] omitted, so 'select' uses arguments passed to function. 11 do 12 echo 13 echo "Your favorite veggie is $vegetable." 14 echo "Yuck!" 15 echo 16 break 17 done 18 } 19 20 choice_of beans rice carrots radishes tomatoes spinach 21 # $1 $2 $3 $4 $5 $6 22 # passed to choice_of() function 23 24 exit 0See also Example 35-3.