Script with parameter

Question

I supossed to make a script that given an number it count to 0, I managed to do this and it's working:

#!/bin/bash
echo -n "type a number: "
read number; echo
while [ $number -ge 0 ]; do
echo -n "$number"
number=$((number-1))
done
echo

Well, I changed it because I need to pass the number by an parameter ex: "./script 5" and it must show the countdown till 0, but its getting in looping. I kind new on all it script/stack what Im doing wrong?

#!/bin/bash
if [ "$*" = "" ]; then
echo
echo "not correct"
echo "must be a int number"
echo
exit
fi

while [ "$1" -ge 0 ]; do
echo "$1"
cont='expr $1-1' 
done
echo

I'm not sure that an issue that surrounds modifying a different variable than the one you test to determine whether to terminate your loop is an issue likely to be helpful to others, and thus a useful knowledgebase addition. — Charles Duffy
– Charles Duffy, Commented Dec 6, 2015 at 16:34

Charles Duffy · Accepted Answer · 2015-12-06 18:11:34Z

You're always using [ "$1" -ge 0 ] as your condition, but the value you actually modify/update is cont, not $1. (Moreover, you modify it based on the value of $1, which isn't changing, so you only ever set $cont to one less than the original value of $1).

Consider:

#!/bin/bash
[[ $1 ]] || { printf '%s\n' "First argument must be an integer" >&2; exit 1; }
for ((i=$1; i>=0; i--)); do
    echo "$i"
done

...and note, among the various changes:

We're consistently referring to the first argument passed as $1, rather than also sometimes referring to it as $*
When we select a variable to modify ($i, here, rather than $cont), we use that same variable in our tests, and also as the source for modification in the loop.
Using expr for math is antiquated; POSIX sh allows $(( )) to create a math context, and bash extends this to also allow C-style for loops in a math context.

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