value - bash script check for option




Check existence of input argument in a Bash shell script (6)

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

((!$#)) && echo No arguments 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 arguments 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.

I need to check the existence of an input argument. I have the following script

if [ "$1" -gt "-1" ]
  then echo hi
fi

I get

[: : integer expression expected

How do I check the input argument1 first to see if it exists?


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.


If you'd like to check if the argument exists, you can check if the # of arguments is greater than or equal to your target argument number.

The following script demonstrates how this works

test.sh

#!/usr/bin/env bash

if [ $# -ge 3 ]
then
  echo script has at least 3 arguments
fi

produces the following output

$ ./test.sh
~
$ ./test.sh 1
~
$ ./test.sh 1 2
~
$ ./test.sh 1 2 3
script has at least 3 arguments
$ ./test.sh 1 2 3 4
script has at least 3 arguments

If you're only interested in detecting 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.

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.


Only because there's a more base point to point out I'll add that you can simply test your string is null:

if [ "$1" ]; then
  echo yes
else
  echo no
fi

Likewise if you're expecting arg count just test your last:

if [ "$3" ]; then
  echo has args correct or not
else
  echo fixme
fi

and so on with any arg or var





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