Shell Programming and Scripting

case $1 in
Jan)
     ;;
Feb)
     ;;
.
.
.
Dec)
      ;;
*)
   echo "Invalid input. Should be one of the following."
   echo "Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec"
   ;;
esac

echo $1 |egrep '^(Jan|Feb|Mar|Apr|may|Jun|Jul|Aug|Sep|Oct|Nov|Dec)$' || echo "Invalid Month entered";

if [[ $1 != @(Jan|Feb|Mar|Apr|May|Jun|Jul|Aug|Sep|Oct|Nov|Dec) ]]
then
   echo "Invalid input. Should be one of the following."
   echo "Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec"
fi

~$ months=( Jan Feb Mar Apr May Jun )
~$ input=Feb
~$ for i in ${months[@]}; do echo $i; done | grep -c $input
1
~$ input=other
~$ for i in ${months[@]}; do echo $i; done | grep -c $input
0

echo $1 | awk '/^(Jan|Feb|Mar|Apr|may|Jun|Jul|Aug|Sep|Oct|Nov|Dec)$/ {print "Invalid Month entered"}'

awk '/^(Jan|Feb|Mar|Apr|may|Jun|Jul|Aug|Sep|Oct|Nov|Dec)$/ {print "Invalid Month entered"}' <<< $1

Months="Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec"
m=Jun
[ "$Months" != "${Months/$m}" ] && echo in || echo out
in
m=Jux
[ "$Months" != "${Months/$m}" ] && echo in || echo out
out