Try:
#!/bin/bash
if [ "$#" -eq "0" ]
then
echo "No arguments supplied"
else
echo "Hello world"
fi
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.
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 [ "$3" ]; then
echo has args correct or not
else
echo fixme
fi
and so on with any arg or var
Another way to detect if arguments were passed to the script:
((!$#)) && 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.
In my case the only working solution to check if last arguments exists is:
if [[ "$7" == '' ]] ; then
echo "error"
exit
fi
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.
I often use this snippet for simple scripts:
#!/bin/bash
if [ -z "$1" ]; then
echo -e "\nPlease call '$0 <argument>' to run this command!\n"
exit 1
fi
one liner bash function validation
myFunction() {
: ${1?"forgot to supply an argument"}
if [ "$1" -gt "-1" ]; then
echo hi
fi
}
add function name and usage
myFunction() {
: ${1?"forgot to supply an argument ${FUNCNAME[0]}() Usage: ${FUNCNAME[0]} some_integer"}
if [ "$1" -gt "-1" ]; then
echo hi
fi
}
add validation to check if integer
to add additional validation, for example to check to see if the argument passed is an integer, modify the validation one liner to call a validation function:
: ${1?"forgot to supply an argument ${FUNCNAME[0]}() Usage: ${FUNCNAME[0]} some_integer"} && validateIntegers $1 || die "Must supply an integer!"
then, construct a validation function that validates the argument, returning 0 on success, 1 on failure and a die function that aborts script on failure
validateIntegers() {
if ! [[ "$1" =~ ^[0-9]+$ ]]; then
return 1 # failure
fi
return 0 #success
}
die() { echo "$*" 1>&2 ; exit 1; }
Even simpler - just use set -u
set -u
makes sure that every referenced variable is set when its used, so just set it and forget it
myFunction() {
set -u
if [ "$1" -gt "-1" ]; then
echo hi
fi
}
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
#!/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
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.Source: Stackoverflow.com