How do I test if a variable is a number in Bash?

I just can't figure out how do I make sure an argument passed to my script is a number or not.

All I want to do is something like this:

test *isnumber* $1 && VAR=$1 || echo "need a number"

Any help?

---

One approach is to use a regular expression, like so:

re='^[0-9]+$'
if ! [[ $yournumber =~ $re ]] ; then
   echo "error: Not a number" >&2; exit 1
fi

If the value is not necessarily an integer, consider amending the regex appropriately; for instance:

^[0-9]+([.][0-9]+)?$

...or, to handle negative numbers:

^-?[0-9]+([.][0-9]+)?$

---

Without bashisms (works even in the System V sh),

case $string in
    ''|*[!0-9]*) echo bad ;;
    *) echo good ;;
esac

This rejects empty strings and strings containing non-digits, accepting everything else.

Negative or floating-point numbers need some additional work. An idea is to exclude - / . in the first "bad" pattern and add more "bad" patterns containing the inappropriate uses of them ( ?*-* / *.*.* )

---

The following solution can also be used in basic shells such as Bourne without the need for regular expressions. Basically any numeric value evaluation operations non-numbers will result in an error which will be implicitly considered as false in shell:

"$var" -eq "$var"

as in:

#!/bin/bash

var=a

if [ "$var" -eq "$var" ] 2>/dev/null; then
  echo number
else
  echo not a number
fi

You can can also test for $? the return code of the operation which is more explicit:

"$var" -eq "$var" 2>/dev/null
if [ $? -ne 0 ]; then
   echo $var is not number
fi

Redirection of standard error is there to hide the "integer expression expected" message that bash prints out in case we do not have a number.

CAVEATS (thanks to the comments below):

Numbers with decimal points are not identified as valid "numbers"

Using [[ ]] instead of [ ] will always evaluate to true

Most non-Bash shells will always evaluate this expression as true

The behavior in Bash is undocumented and may therefore change without warning

If value includes spaces after number (eg "1 a") produces error, like bash: [[: 1 a: syntax error in expression (error token is "a")

If value is the same as var-name (eg i="i"), produces error, like bash: [[: i: expression recursion level exceeded (error token is "i")

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