Bash how to detect missing mandatory arguments passed to a script

Question

I have a Bash script and I need to pass a certain number of arguments to make it work.

./upload.sh $ARG1 $ARG2 $ARG3

Let's say that the 2 mandatory fields are ARG1 and ARG2, and
ARG1 and 3 are not empty.

I think the script will run and think that it has the 2 mandatory arguments, Is there a way to detect that ARG2 is missing/empty? I need to return exit 1 and not exit 0.

Here's a bit of the script

RESOURCE=$1
CONTAINER=$2
APP_NAME=$3

if [[ -z $RESOURCE || -z $CONTAINER ]];
then
    echo `date`" - Missing mandatory arguments: resource and container. "
    echo `date`" - Usage: ./upload.sh  [resource] [container] [appname] . "
    exit 1
fi

Thanks in advance,

Alain

Answer 1

I like:

VARNAME=${1:?"missing arg 1 for VARNAME"}

Answer 2

I always use a little function to check all the arguments:

process_arguments() {
    while [ -n "$1" ]
    do
        case $1 in
            -h|--help) echo "some usage details"; exit 1;;
            -x) do_something; shift; break;;
            -y) do_something_else; shift; break;;
            *) echo "some usage details"; exit 1;;
        esac
        echo $1; shift
    done
}

This way if you miss anything, the proper usage details will be displayed the script will exit. Otherwise, you can enter the args in any order. When your script starts, just call

process_arguments "$@"

and you should be all set.

Answer 3

Is there a way to detect that ARG2 is missing/empty?

NO. The way you're passing the arguments, those would be interpreted as arguments 1 and 2.

You could instead say:

./upload.sh "$ARG1" "$ARG2" "$ARG3"

in order to make bash interpret the arguments correctly whether those are empty or not.

---

Example:

$ cat file
[ -z $1 ] && echo "Arg 1 missing"
[ -z $2 ] && echo "Arg 2 missing"
[ -z $3 ] && echo "Arg 3 missing"
$ bash file "$HOME" "$FOOBAR" "$USER"
Arg 2 missing

(The variable FOOBAR was undefined in the above example.)

Answer 4

Args as you are using them are positional so is you only provided 1 and 3, but 1 and 2 are require, you just accidentally provided a value for arg 3 as arg 2.

You could require that your optional arguments start with a dash -, and if you find either $1 or $2 start with a dash you know you are missing a required. You of course would have to remove the dash from $3.

A better way to handle args is to use getopts. Here are a couple of resources to get you started:

• http://wiki.bash-hackers.org/howto/getopts_tutorial
• http://rsalveti.wordpress.com/2007/04/03/bash-parsing-arguments-with-getopts/

This will allow you to handle args in a way similar to standard commandline utils. If you write commandline utils in C you have probably used the C library version of getops before.