BASH: check if the first argument match a number

cristalp · 11-14-2012, 09:18 AM

I am writing a bash script and I need to check if the first argument matchs a number. The number can be either a decimal number or a real number, no mater how many decimal places or how large it is. 37.987 or 956 or 1 or 3.14159, anything like that.

How can I state this condition in if [[ ... ]] ?

Thanks for your help!

catkin · 11-14-2012, 09:44 AM

The classic bash test for an integer is [[ $var =~ ^[0-9]+$ ]]

That needs to be extended to accommodate a possible decimal point: [[ $1 =~ ^([0-9]*\.)?[0-9]+$ ]]

It works on the posted numbers but would not work on negative numbers or numbers with thousands separators such as 1,000. OK?

colucix · 11-14-2012, 10:28 AM

A non-bash solution using awk

Code:

if echo $number | awk '$0 != $0 + 0 { exit 1 }'
then
  echo $number is a number
else
  echo $number is not a number
fi

This manages floating-point numbers, negative numbers and scientific notation. I'm not sure about thousand-separators.

michaelk · 11-14-2012, 10:45 AM

Is this is for a homework assignment?

Essentially bash variables are strings. I assume you are trying to match an exact number and therefore it is just a string comparison.
if [[ "$var" == "value" ]] then

BTW bash can only handle integer math. If you need some floating point math capability then bc is one option.

http://linux.about.com/od/commands/l/blcmdl1_bc.htm

grail · 11-15-2012, 07:59 AM

@michaelk - I find the following to be a curious statement:

Quote:

Essentially bash variables are strings.

I am not sure how you came up with this?

Allow me to demonstrate why I think it is incorrect:

Code:

$ cat string_and_num_test.sh
#!/bin/bash

s='100'
n=100

[[ $x < "12" ]] && echo true for string

(( n < 12 )) && echo true for num

$ ./string_and_num_test.sh
true for string

We see the test works for the string as characters are compared from left to right however the number test clearly shows that it is being calculated as a number.

Now unless bash does some magic to number tests compared to string ones I would say you can have both string and numerical variables.

michaelk · 11-15-2012, 11:10 AM

http://www.tldp.org/LDP/abs/html/untyped.html