4. Examples

1. Let's see an example program that validates its input:

   validate.sh

   #!/bin/bash
   
   # read-validate: validate input
   
   invalid_input () {
       echo "Invalid input '$REPLY'" >&2
       exit 1
   }
   
   read -p "Enter a single item > "
   
   # input is empty (invalid)
   [[ -z "$REPLY" ]] && invalid_input
   
   # input is multiple items (invalid)
   (( "$(echo "$REPLY" | wc -w)" > 1 )) && invalid_input
   
   # is input a valid filename?
   if [[ "$REPLY" =~ ^[-[:alnum:]\._]+$ ]]; then
       echo "'$REPLY' is a valid filename."
       if [[ -e "$REPLY" ]]; then
           echo "And file '$REPLY' exists."
       else
           echo "However, file '$REPLY' does not exist."
       fi
   
       # is input a floating point number?
       if [[ "$REPLY" =~ ^-?[[:digit:]]*\.[[:digit:]]+$ ]]; then
           echo "'$REPLY' is a floating point number."
       else
           echo "'$REPLY' is not a floating point number."
       fi
   
       # is input an integer?
       if [[ "$REPLY" =~ ^-?[[:digit:]]+$ ]]; then
           echo "'$REPLY' is an integer."
       else
           echo "'$REPLY' is not an integer."
       fi
   else
       echo "The string '$REPLY' is not a valid filename."
   fi

   vim validate.sh

   Try it a few times with different inputs:

   ./validate.sh

2. Let's see a menu driven example program:

   menu.sh

   #!/bin/bash
   
   # read-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] > "
   
   if [[ ! "$REPLY" =~ ^[0-3]$ ]]; then
       echo "Invalid entry." >&2
       exit 1
   fi
   
   if [[ "$REPLY" == 0 ]]; then
       echo "Program terminated."
       exit
   fi
   
   if [[ "$REPLY" == 1 ]]; then
       echo "Hostname: $HOSTNAME"
       uptime
       exit
   fi
   
   if [[ "$REPLY" == 2 ]]; then
       df -h .
       exit
   fi
   
   if [[ "$REPLY" == 3 ]]; then
       if [[ "$(id -u)" -eq 0 ]]; then
           echo "Home Space Utilization (All Users)"
           du -sh /home/*
       else
           echo "Home Space Utilization ($USER)"
           du -sh "$HOME"
       fi
       exit
   fi

   vim menu.sh

   Notice the use of the exit command in this script. It is used here to prevent the script from executing unnecessary code after an action has been carried out.

   Try if a few times.

3. As an exercise, try to modify these examples so that instead of [[... ]] and ((...), they use the command test.

   Hint: Use grep to evaluate the regular expressions and evaluate the exit status.

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