1. Looping with while and until

The syntax of the while command is as follows:

while commands; do commands; done

1. A simple example:

   while-count.sh

   #!/bin/bash
   
   # while-count: display a series of numbers
   
   count=1
   
   while [[ "$count" -le 5 ]]; do
       echo "$count"
       count=$((count + 1))
   done
   echo "Finished."

   vim while-count.sh

   ./while-count.sh

2. We can use a while loop to improve the menu program from the previous lesson:

   while-menu.sh

   #!/bin/bash
   
   # while-menu: a menu driven system information program
   
   DELAY=3 # Number of seconds to display results
   
   while [[ "$REPLY" != 0 ]]; do
       clear
       cat <<- _EOF_
   	Please Select:
   	1. Display System Information
   	2. Display Disk Space
   	3. Display Home Space Utilization
   	0. Quit
   	_EOF_
       read -p "Enter selection [0-3] > "
   
       if [[ "$REPLY" =~ ^[0-3]$ ]]; then
           if [[ $REPLY == 1 ]]; then
               echo "Hostname: $HOSTNAME"
               uptime
               sleep "$DELAY"
           fi
           if [[ "$REPLY" == 2 ]]; then
               df -h .
               sleep "$DELAY"
           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
               sleep "$DELAY"
           fi
       else
           echo "Invalid entry."
           sleep "$DELAY"
       fi
   done
   echo "Program terminated."

   vim while-menu.sh

   By enclosing the menu in a while loop, we are able to have the program repeat the menu display after each selection. The loop continues as long as REPLY is not equal to 0 and the menu is displayed again, giving the user the opportunity to make another selection. At the end of each action, a sleep command is executed so the program will pause for a few seconds to allow the results of the selection to be seen before the screen is cleared and the menu is redisplayed. Once REPLY is equal to 0, indicating the “quit” selection, the loop terminates and execution continues with the line following done.

   ./while-menu.sh

3. Inside a loop in bash we can use break and continue.

   while-menu2.sh

   #!/bin/bash
   
   # while-menu: a menu driven system information program
   
   DELAY=3 # Number of seconds to display results
   
   while true; do
       clear
       cat <<- _EOF_
   	Please Select:
   	1. Display System Information
   	2. Display Disk Space
   	3. Display Home Space Utilization
   	0. Quit
   	_EOF_
       read -p "Enter selection [0-3] > "
   
       if [[ "$REPLY" == 0 ]]; then
           break
       fi
       if [[ ! "$REPLY" =~ ^[0-3]$ ]]; then
           echo "Invalid entry."
           sleep "$DELAY"
           continue
       fi
       if [[ $REPLY == 1 ]]; then
           echo "Hostname: $HOSTNAME"
           uptime
           sleep "$DELAY"
           continue
       fi
       if [[ "$REPLY" == 2 ]]; then
           df -h .
           sleep "$DELAY"
           continue
       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
           sleep "$DELAY"
           continue
       fi
   done
   echo "Program terminated."

   vim while-menu2.sh

   ./while-menu2.sh

4. The until loop is very similar to the while loop, but with a negated condition.

   until-count.sh

   #!/bin/bash
   
   # until-count: display a series of numbers
   
   count=1
   
   until [[ "$count" -gt 5 ]]; do
       echo "$count"
       count=$((count + 1))
   done
   echo "Finished."

   vim until-count.sh

   ./until-count.sh

5. We can also read the standard input with while and until:

   cat distros.txt
   while-read.sh

   #!/bin/bash
   
   # while-read: read lines from a file
   
   while read distro version release
   do
       printf "Distro: %s\tVersion: %s\tReleased: %s\n" \
           "$distro" \
           "$version" \
           "$release"
   done < distros.txt

   vim while-read.sh

   The while loop will continue as long as the read command is successful getting input from stdin, and we redirect stdin to get data from the file distros.txt (by using the operator < at the end of the while command).

   ./while-read.sh

   We can also use a pipe (|) to redirect the stdin:

   while-read2.sh

   #!/bin/bash
   
   # while-read2: read lines from a file
   
   sort -k 1,1 -k 2n distros.txt | \
       while read distro version release; do
           printf "Distro: %s\tVersion: %s\tReleased: %s\n" \
               "$distro" \
               "$version" \
               "$release"
       done

   vim while-read2.sh

   Notice that we are breaking long commands by adding a \ at the end of a line, in order to make the program more readable and clear.

   ./while-read2.sh

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