Count down script is not working

Question

I am trying to create a script that will count down to 0 from what ever number I give it. Below is my script and basically nothing happens, no error message, I merely get the standard command line prompt back.

#!/bin/bash
#countdown
#counts down to 0 from whatever number you give it
#displaying a number each second

NUM=${1:-0}
if [ $NUM -gt 0 ]
then
   while [ $NUM -gt 0 ]
   do
      if [ -f /usr/bin/banner ]
      then
         /usr/bin/banner "$NUM"
      else
         echo $NUM
      fi
         NUM=$(($NUM-1))
         sleep 2
   done
fi

Answer 1

#!/bin/bash

printf "Type an integer number: " && read NUM
if [ $NUM -gt 0 ]
then
   while [ $NUM -ge 0 ]
   do
      if [ -f /usr/bin/banner ]
      then
         /usr/bin/banner "$NUM"
      else
         echo $NUM
      fi
         NUM=$(($NUM-1))
         sleep 2
   done
fi

output:

:~$ ./countdown.sh 
Type an integer number: 10
10
9
8
7
6
5
4
3
2
1
0

explanation:
⠀1. line 3 prompts the user to input an integer number and reads it into the variable NUM.
⠀2. Changed the -gt in line 6 to -ge so that it counts down to zero.
⠀3. The output is displayed in banner format if sysvbanner is installed or else as text if it isn't.

Answer 2

Improved and commented code:

#!/bin/bash

num=${1:-undefined}                                   # If $1 (the first argument passed to the script) is set, then num=$1, else num=undefined.
cmd=$(which {banner,echo} | head -1 | xargs basename) # If banner is installed, then cmd=baner, else cmd=echo.

until [[ "$num" =~ ^[0-9]+$ ]]; do                    # Until $num become a valid number (loop will not be executed if $1 is set):
    read -p "Type a number: " num                         # Ask the user for a valid number.
done                                                  # End of the until loop.

for ((num;num>=0;num--)); do                          # Loop using $num as variable; while $num is greater or equal than zero; num=$num-1.
   $cmd $num                                              # Runs $cmd (banner or echo) passing $num as argument.
   sleep 1                                                # Stop the program execution for one second.
done                                                  # End of the for loop.

The above code will include zero in the countdown, if do you want to stop when the countdown reaches 1, then you only need to make a few changes:

1. In the 6th line, change ^[0-9]+$ by ^[1-9]+[0-9]*$ so it looks like this:

   until [[ "$num" =~ ^[1-9]+[0-9]*$ ]]; do              # Until $num become a valid number (loop will not be executed if $1 is set):
   

2. In the 10th line, remove the = sign so it looks like this (I've updated also the comment):

   for ((num;num>0;num--)); do                           # Loop using $num as variable; while $num is strictly greater than zero; num=$num-1.
   

---

Your original program doesn't work because:

• You not passed a number as argument to the program.
• A example invocation of this command is ./countdown 5 where 5 is the number.

• If do you want to handle this, you can add a else to your code (look a the five last lines):

  #!/bin/bash
  #countdown
  #counts down to 0 from whatever number you give it
  #displaying a number each second
  
  NUM=${1:-0}
  if [ $NUM -gt 0 ]
  then
     while [ $NUM -gt 0 ]
     do
        if [ -f /usr/bin/banner ]
        then
           /usr/bin/banner "$NUM"
        else
           echo $NUM
        fi
           NUM=$(($NUM-1))
           sleep 2
     done
  else
      echo "Error: number not specified."
      echo "Usage: $0 <number>"
      exit 1
  fi
  

---

NUM=${1:-0} means:

${PARAMETER:-WORD}

If the parameter PARAMETER is unset (never was defined) or null (empty), this one expands to WORD, otherwise it expands to the value of PARAMETER, as if it just was ${PARAMETER}.

echo "Your home directory is: ${HOME:-/home/$USER}."
echo "${HOME:-/home/$USER} will be used to store your personal data."

If HOME is unset or empty, everytime you want to print something useful, you need to put that parameter syntax in.

Source: http://wiki.bash-hackers.org/syntax/pe#use_a_default_value

In your case, it means that, if you passed a argument to the script, NUM will be equal to that argument, else, NUM will be equal to 0

Answer 3

The NUM=${1:-0} line means that the variable NUM is set to $1 if a parameter is passed to the script, and to 0 if no parameter is passed at all. That explains why you have no output at all; the threshold is always set to 0 if the script is executed without passing a parameter to it, e.g.:

bash <script_name>

_{*<script_name> = name of your bash script;}

or:

./<script_name>

_{*<script_name> = name of your bash script;}

So you what you really need to do is to just pass the threshold number to your script upon execution, i.e.:

bash <script_name> <threshold_number>

_{*<script_name> = name of your bash script; <number> = thresold number}

or:

./<script_name> <threshold_number>

_{*<script_name> = name of your bash script; <number> = thresold number}