bash shell script check input argument

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.

Another way to detect if arguments were passed to the script:

((!$#)) && echo No arugments supplied!

Note that (( expr )) causes the expression to be evaluated as per rules of Shell Arithmetic.

In order to exit in the absence of any arguments, one can say:

((!$#)) && echo No arugments supplied! && exit 1

Another (analogous) way to say the above would be:

let $# || echo No arguments supplied

let $# || { echo No arguments supplied; exit 1; }  # Exit if no arguments!

help let says:

let: let arg [arg ...]

  Evaluate arithmetic expressions.

  ...

  Exit Status:
  If the last ARG evaluates to 0, let returns 1; let returns 0 otherwise.

If you're only interested in bailing if a particular argument is missing, Parameter Substitution is great:

#!/bin/bash
# usage-message.sh

: ${1?"Usage: $0 ARGUMENT"}
#  Script exits here if command-line parameter absent,
#+ with following error message.
#    usage-message.sh: 1: Usage: usage-message.sh ARGUMENT

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.

Try,

 #!/bin/bash
 if [ "$#" -eq  "0" ]
   then
 echo "No arugments supplied"
 else
 echo "Hello world"
 fi

Thanks.

as a small reminder the numeric test operators in bash only work on integers (-eq/-lt/-ge etc)

I like to ensure my $vars are ints by

var=$(( var + 0 ))

before I test them, just to defend against the "[: integer arg required" error