empty option output error

Shell Programming and Scripting
1

I have a script (multirun.sh) which launches the program bsim_em.x or bsim_es.x depending on the value entered from the screen:

> multirun.sh 1 (executes bsim_em.x)
> multirun.sh 2 (executes bsim_es.x)

which, simplifying, I do with the following lines in the multirun.sh script:

if [ "$1" = "1" ]
then
./bsim_em.x
fi

if [ "$1" = "2" ]
then
./bsim_em.x
fi

how could I write an output error if $1 is different from 1,2 or is not defined? Thanks in advanced

2 
case "$1" in
   1) ./bsim_em.x ;;
   2) ./bsim_es.x  ;;
  "") echo "Missing parameter!" ;;
   *) echo "Invalid parameter!" ;;
esac

Jean-Pierre.

3

well, I found out it is just

if [ "$1" = "" ]
then
echo 'esto es 2'
fi

4

Thanks a lot Jean-Pierre. Much better!