2. Branching with case

The command case is a multiple-choice command.

1. Let's see an example that implements a menu program with case:

   case-menu.sh

   #!/bin/bash
   
   # case-menu: a menu driven system information program
   
   clear
   echo "
   Please Select:
   
   1. Display System Information
   2. Display Disk Space
   3. Display Home Space Utilization
   0. Quit
   "
   read -p "Enter selection [0-3] > "
   
   case "$REPLY" in
       0)  echo "Program terminated."
           exit
           ;;
       1)  echo "Hostname: $HOSTNAME"
           uptime
           ;;
       2)  df -h .
           ;;
       3)  if [[ "$(id -u)" -eq 0 ]]; then
               echo "Home Space Utilization (All Users)"
               du -sh /home/*
           else
               echo "Home Space Utilization ($USER)"
               du -sh "$HOME"
           fi
           ;;
       *)  echo "Invalid entry" >&2
           exit 1
           ;;
   esac

   vim case-menu.sh

   We have seen this example before, implemented with if and it is clear that with case it is much simpler.

   ./case-menu.sh

   case attempts a match against the specified patterns. When a match is found, the commands associated with the specified pattern are executed. After a match is found, no further matches are attempted.

2. The patterns used by case are the same as those used by pathname expansion. For example:

   • a) -- matches the character "a"
   • [[:alpha:]]) -- matches any alphabetic character
   • ???) -- matches 3 characters
   • *.txt) -- matches anything that ends in .txt
   • *) -- matches anything

   It is good practice to include *) as the last pattern in a case command, to catch any values that did not match a previous pattern.

   Let's see an example script with patterns:

   case-patterns.sh

   #!/bin/bash
   
   read -p "enter word > "
   
   case "$REPLY" in
       [[:alpha:]])  echo "it is a single alphabetic character" ;;
       [ABC][0-9])   echo "it is A, B, or C followed by a digit" ;;
       ???)          echo "it is three characters long" ;;
       *.txt)        echo "it is a word ending in '.txt'" ;;
       *)            echo "it is something else" ;;
   esac

   vim case-patterns.sh

   ./case-patterns.sh    # enter: x

   ./case-patterns.sh    # enter: B2

   ./case-patterns.sh    # enter: foo.txt

   ./case-patterns.sh    # enter: xyz

   ./case-patterns.sh    # enter: ab

3. It is also possible to combine multiple patterns using the vertical bar character as a separator. Let's see a modified menu program that uses letters instead of digits for menu selection:

   case-menu-l.sh

   #!/bin/bash
   
   # case-menu: a menu driven system information program
   
   clear
   echo "
   Please Select:
   
   A. Display System Information
   B. Display Disk Space
   C. Display Home Space Utilization
   Q. Quit
   "
   read -p "Enter selection [A, B, C or Q] > "
   
   case "$REPLY" in
       q|Q)  echo "Program terminated."
             exit
             ;;
       a|A)  echo "Hostname: $HOSTNAME"
             uptime
             ;;
       b|B)  df -h .
             ;;
       c|C)  if [[ "$(id -u)" -eq 0 ]]; then
                 echo "Home Space Utilization (All Users)"
                 du -sh /home/*
             else
                 echo "Home Space Utilization ($USER)"
                 du -sh "$HOME"
             fi
             ;;
       *)    echo "Invalid entry" >&2
             exit 1
             ;;
   esac

   vim case-menu-l.sh

   Notice how the new patterns allow for entry of both uppercase and lowercase letters.

   ./case-menu-l.sh

4. When a pattern is matched, the corresponding actions are executed, and ;; makes sure that processing is stopped (without trying to match the following patterns). If we want instead to try matching them as well, we can use ;;& instead, as in this example:

   case4.sh

   #!/bin/bash
   # case4: test a character
   
   # -n 1:  read only one char and don't wait for enter to be pressed
   read -n 1 -p "Type a character > "
   echo
   case "$REPLY" in
       [[:upper:]])    echo "'$REPLY' is upper case."             ;;&
       [[:lower:]])    echo "'$REPLY' is lower case."             ;;&
       [[:alpha:]])    echo "'$REPLY' is alphabetic."             ;;&
       [[:digit:]])    echo "'$REPLY' is a digit."                ;;&
       [[:graph:]])    echo "'$REPLY' is a visible character."    ;;&
       [[:punct:]])    echo "'$REPLY' is a punctuation symbol."   ;;&
       [[:space:]])    echo "'$REPLY' is a whitespace character." ;;&
       [[:xdigit:]])   echo "'$REPLY' is a hexadecimal digit."    ;;&
   esac

   vim case4.sh

   ./case4.sh    # enter: a

   ./case4.sh    # enter: X

   ./case4.sh    # enter: +

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