How to detect even or odd minutes in bash script

Question

I need to detect whether the current minutes are even or odd regardless of the time zone. I tried this according to answers from several sites:

#!/bin/bash 
n=$(date +"%M")
r=`expr$n%2`
if [ r -eq 0 ] 
then 
echo "Even minute"
else 
echo "Odd minute"
 fi

When I run the script I get

./Odd_even.sh: line 5: expr04%2: command not found
./Odd_even.sh: line 6: [: r: integer expression expected – 

What is wrong here?

Answer 1

Working with hours or minutes as numbers in bash has a nasty edge case: bash treats numbers with a leading zero as octal, and 08 and 09 are invalid octal numbers:

min=09
echo $(( min % 2 == 0 ))

bash: 09: value too great for base (error token is "09")

Use %-M as the format directive -- it will return the minutes with no leading zero.

---

Additionally:

1. bash can do arithmetic, you don't need to call out to expr

   if (( n % 2 == 0 )); then
     echo "$n is even"
   else
     echo "$n is odd"
   fi
   

2. and bash can do strftime-like things. The %()T directive in bash's builtin printf allows the date format string inside the parentheses. In this simple case, you don't need to call out to date.

   minute=$(printf '%(%-M)T' -1)   # -1 is a special value that means "now"
   # or even
   printf -v minute '%(%-M)T' -1