Check existence of input argument in a Bash shell script

It is:

if [ $# -eq 0 ]
  then
    echo "No arguments supplied"
fi

The $# variable will tell you the number of input arguments the script was passed.

Or you can check if an argument is an empty string or not like:

if [ -z "$1" ]
  then
    echo "No argument supplied"
fi

The -z switch will test if the expansion of "$1" is a null string or not. If it is a null string then the body is executed.

It is better to demonstrate this way

if [[ $# -eq 0 ]] ; then
    echo 'some message'
    exit 1
fi

You normally need to exit if you have too few arguments.

In some cases you need to check whether the user passed an argument to the script and if not, fall back to a default value. Like in the script below:

scale=${2:-1}
emulator @$1 -scale $scale

Here if the user hasn't passed scale as a 2nd parameter, I launch Android emulator with -scale 1 by default. ${varname:-word} is an expansion operator. There are other expansion operators as well:

• ${varname:=word} which sets the undefined varname instead of returning the word value;
• ${varname:?message} which either returns varname if it's defined and is not null or prints the message and aborts the script (like the first example);
• ${varname:+word} which returns word only if varname is defined and is not null; returns null otherwise.