[linux] 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?

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 (?*-* / *.*.*)

This tests if a number is a non negative integer and is both shell independent (i.e. without bashisms) and uses only shell built-ins:

INCORRECT.

As this first answer (below) allows for integers with characters in them as long as the first are not first in the variable.

[ -z "${num##[0-9]*}" ] && echo "is a number" || echo "is not a number";

CORRECT .

As jilles commented and suggested in his answer this is the correct way to do it using shell-patterns.

[ ! -z "${num##*[!0-9]*}" ] && echo "is a number" || echo "is not a number";

Nobody suggested bash's extended pattern matching:

[[ $1 == ?(-)+([0-9]) ]] && echo "$1 is an integer"

or using new POSIX style:

[[ $1 == ?(-)+([:digit:]) ]] && echo "$1 is an integer"

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 using 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 [ -n "$var" ] && [ "$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:

[ -n "$var" ] && [ "$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 the value includes spaces after the number (e.g. "1 a") produces error, like bash: [[: 1 a: syntax error in expression (error token is "a")
• If the value is the same as var-name (e.g. i="i"), produces error, like bash: [[: i: expression recursion level exceeded (error token is "i")

I found quite a short version:

function isnum()
{
    return `echo "$1" | awk -F"\n" '{print ($0 != $0+0)}'`
}

Quick & Dirty: I know it's not the most elegant way, but I usually just added a zero to it and test the result. like so:

function isInteger {
  [ $(($1+0)) != 0 ] && echo "$1 is a number" || echo "$1 is not a number"
 }

x=1;      isInteger $x
x="1";    isInteger $x
x="joe";  isInteger $x
x=0x16 ;  isInteger $x
x=-32674; isInteger $x   

$(($1+0)) will return 0 or bomb if $1 is NOT an integer. for Example:

function zipIt  { # quick zip - unless the 1st parameter is a number
  ERROR="not a valid number. " 
  if [ $(($1+0)) != 0 ] ; then  # isInteger($1) 
      echo " backing up files changed in the last $1 days."
      OUT="zipIt-$1-day.tgz" 
      find . -mtime -$1 -type f -print0 | xargs -0 tar cvzf $OUT 
      return 1
  fi
    showError $ERROR
}

NOTE: I guess I never thought to check for floats or mixed types that will make the entire script bomb... in my case, I didn't want it go any further. I'm gonna play around with mrucci's solution and Duffy's regex - they seem the most robust within the bash framework...

http://tldp.org/LDP/Bash-Beginners-Guide/html/sect_04_03.html

You can also use bash's character classes.

if [[ $VAR = *[[:digit:]]* ]]; then
 echo "$VAR is numeric"
else
 echo "$VAR is not numeric"
fi

Numerics will include space, the decimal point, and "e" or "E" for floating point.

But, if you specify a C-style hex number, i.e. "0xffff" or "0XFFFF", [[:digit:]] returns true. A bit of a trap here, bash allows you do to something like "0xAZ00" and still count it as a digit (isn't this from some weird quirk of GCC compilers that let you use 0x notation for bases other than 16???)

You might want to test for "0x" or "0X" before testing if it's a numeric if your input is completely untrusted, unless you want to accept hex numbers. That would be accomplished by:

if [[ ${VARIABLE:1:2} = "0x" ]] || [[ ${VARIABLE:1:2} = "0X" ]]; then echo "$VAR is not numeric"; fi

This is a little rough around the edges but a little more novice friendly.

if [ $number -ge 0 ]
then
echo "Continue with code block"
else
echo "We matched 0 or $number is not a number"
fi

This will cause an error and print "Illegal number:" if $number is not a number but it will not break out of the script. Oddly there is not a test option I could find to just test for an integer. The logic here will match any number that is greater than or equal to 0.

Stack popped a message asked me if I really want to answer after 30+ answers? But of course!!! Use bash new features and here it is: (after the comment I made a change)

function isInt() { ([[ $1 -eq $(( $1 + 0 )) ]] 2>/dev/null && [[ $1 != '' ]] && echo 1) || echo '' }

function isInt() {
   ([[ $1 =~ ^[-+0-9]+$  ]] && [[ $1 -eq $(( $1 + 0 )) ]] 2>/dev/null && [[ $1 != '' ]] && echo 1) || echo ''
}

Supports:

===============out-of-the-box==================
 1. negative integers (true & arithmetic),
 2. positive integers (true & arithmetic),
 3. with quotation (true & arithmetic),
 4. without quotation (true & arithmetic),
 5. all of the above with mixed signs(!!!) (true & arithmetic),
 6. empty string (false & arithmetic),
 7. no value (false & arithmetic),
 8. alphanumeric (false & no arithmetic),
 9. mixed only signs (false & no arithmetic),
================problematic====================
 10. positive/negative floats with 1 decimal (true & NO arithmetic),
 11. positive/negative floats with 2 or more decimals (FALSE & NO arithmetic).

True/false is what you get from the function only when used combined with process substitution like in [[ $( isInt <arg> ) ]] as there is no logical type in bash neither return value of function.

I use capital when the result of the test expression is WRONG whereas, it should be the reverse!

By 'arithmetic' I mean bash can do math like in this expression: $x=$(( $y + 34)).

I use 'arithmetic/no arithmetic' when in mathematical expressions the argument acts as it is expected and 'NO arithmetic' when it misbehaves compared with what it is expected.

As you see, only no 10 and 11 are the problematic ones!

Perfect!

PS: Note that the MOST popular answer fails in case 9!

• variable to check

  number=12345 or number=-23234 or number=23.167 or number=-345.234

• check numeric or non-numeric

  echo $number | grep -E '^-?[0-9]*\.?[0-9]*$' > /dev/null

• decide on further actions based on the exit status of the above

  if [ $? -eq 0 ]; then echo "Numeric"; else echo "Non-Numeric"; fi

[[ $1 =~ ^-?[0-9]+$ ]] && echo "number"

Don't forget - to include negative numbers!

Just a follow up to @mary. But because I don't have enough rep, couldn't post this as a comment to that post. Anyways, here is what I used:

isnum() { awk -v a="$1" 'BEGIN {print (a == a + 0)}'; }

The function will return "1" if the argument is a number, otherwise will return "0". This works for integers as well as floats. Usage is something like:

n=-2.05e+07
res=`isnum "$n"`
if [ "$res" == "1" ]; then
     echo "$n is a number"
else
     echo "$n is not a number"
fi

Below is a Script written by me and used for a script integration with Nagios and it is working properly till now

#!/bin/bash
# Script to test variable is numeric or not
# Shirish Shukla
# Pass arg1 as number
a1=$1
a=$(echo $a1|awk '{if($1 > 0) print $1; else print $1"*-1"}')
b=$(echo "scale=2;$a/$a + 1" | bc -l 2>/dev/null)
if [[ $b > 1 ]]
then
    echo "$1 is Numeric"
else
    echo "$1 is Non Numeric"
fi

EG:

# sh isnumsks.sh   "-22.22"
-22.22 is Numeric

# sh isnumsks.sh   "22.22"
22.22 is Numeric

# sh isnumsks.sh   "shirish22.22"
shirish22.22 is Non  Numeric

I would try this:

printf "%g" "$var" &> /dev/null
if [[ $? == 0 ]] ; then
    echo "$var is a number."
else
    echo "$var is not a number."
fi

Note: this recognizes nan and inf as number.

I was looking at the answers and... realized that nobody thought about FLOAT numbers (with dot)!

Using grep is great too.
-E means extended regexp
-q means quiet (doesn't echo)
-qE is the combination of both.

To test directly in the command line:

$ echo "32" | grep -E ^\-?[0-9]?\.?[0-9]+$  
# answer is: 32

$ echo "3a2" | grep -E ^\-?[0-9]?\.?[0-9]+$  
# answer is empty (false)

$ echo ".5" | grep -E ^\-?[0-9]?\.?[0-9]+$  
# answer .5

$ echo "3.2" | grep -E ^\-?[0-9]?\.?[0-9]+$  
# answer is 3.2

Using in a bash script:

check=`echo "$1" | grep -E ^\-?[0-9]*\.?[0-9]+$`

if [ "$check" != '' ]; then    
  # it IS numeric
  echo "Yeap!"
else
  # it is NOT numeric.
  echo "nooop"
fi

To match JUST integers, use this:

# change check line to:
check=`echo "$1" | grep -E ^\-?[0-9]+$`

bash regular expression vs bash parameter expansion

As Charles Duffy's answer work, but only use bash regular expression, and I know this to be slow, I would like to show another way, using only bash parameter expansion:

For positive integers only:

is_num() { [ "$1" ] && [ -z "${1//[0-9]}" ] ;}

For integers:

is_num() { 
    local chk=${1#[+-]};
    [ "$chk" ] && [ -z "${chk//[0-9]}" ]
}

Then for floating:

is_num() { 
    local chk=${1#[+-]};
    chk=${chk/.}
    [ "$chk" ] && [ -z "${chk//[0-9]}" ]
}

To match initial request:

set -- "foo bar"
is_num "$1" && VAR=$1 || echo "need a number"
need a number

set -- "+3.141592"
is_num "$1" && VAR=$1 || echo "need a number"

echo $VAR
+3.141592

Now a little comparission:

There is a same check based on Charles Duffy's answer:

cdIs_num() { 
    local re='^[+-]?[0-9]+([.][0-9]+)?$';
    [[ $1 =~ $re ]]
}

Some tests:

if is_num foo;then echo It\'s a number ;else echo Not a number;fi
Not a number
if cdIs_num foo;then echo It\'s a number ;else echo Not a number;fi
Not a number
if is_num 25;then echo It\'s a number ;else echo Not a number;fi
It's a number
if cdIs_num 25;then echo It\'s a number ;else echo Not a number;fi
It's a number
if is_num 3+4;then echo It\'s a number ;else echo Not a number;fi
Not a number
if cdIs_num 3+4;then echo It\'s a number ;else echo Not a number;fi
Not a number
if is_num 3.1415;then echo It\'s a number ;else echo Not a number;fi
It's a number
if cdIs_num 3.1415;then echo It\'s a number ;else echo Not a number;fi
It's a number

Ok, that's fine. Now how many time will take all this (On my raspberry pi):

time for i in {1..1000};do is_num +3.14159265;done
real    0m2.476s
user    0m1.235s
sys     0m0.000s

Then with regular expressions:

time for i in {1..1000};do cdIs_num +3.14159265;done
real    0m4.363s
user    0m2.168s
sys     0m0.000s

Can't comment yet so I'll add my own answer, which is an extension to glenn jackman's answer using bash pattern matching.

My original need was to identify numbers and distinguish integers and floats. The function definitions deducted to:

function isInteger() {
    [[ ${1} == ?(-)+([0-9]) ]]
}

function isFloat() {
    [[ ${1} == ?(-)@(+([0-9]).*([0-9])|*([0-9]).+([0-9]))?(E?(-|+)+([0-9])) ]]
}

I used unit testing (with shUnit2) to validate my patterns worked as intended:

oneTimeSetUp() {
    int_values="0 123 -0 -123"
    float_values="0.0 0. .0 -0.0 -0. -.0 \
        123.456 123. .456 -123.456 -123. -.456
        123.456E08 123.E08 .456E08 -123.456E08 -123.E08 -.456E08 \
        123.456E+08 123.E+08 .456E+08 -123.456E+08 -123.E+08 -.456E+08 \
        123.456E-08 123.E-08 .456E-08 -123.456E-08 -123.E-08 -.456E-08"
}

testIsIntegerIsFloat() {
    local value
    for value in ${int_values}
    do
        assertTrue "${value} should be tested as integer" "isInteger ${value}"
        assertFalse "${value} should not be tested as float" "isFloat ${value}"
    done

    for value in ${float_values}
    do
        assertTrue "${value} should be tested as float" "isFloat ${value}"
        assertFalse "${value} should not be tested as integer" "isInteger ${value}"
    done

}

Notes: The isFloat pattern can be modified to be more tolerant about decimal point (@(.,)) and the E symbol (@(Ee)). My unit tests test only values that are either integer or float, but not any invalid input.

Old question, but I just wanted to tack on my solution. This one doesn't require any strange shell tricks, or rely on something that hasn't been around forever.

if [ -n "$(printf '%s\n' "$var" | sed 's/[0-9]//g')" ]; then
    echo 'is not numeric'
else
    echo 'is numeric'
fi

Basically it just removes all digits from the input, and if you're left with a non-zero-length string then it wasn't a number.

Following up on David W's answer from Oct '13, if using expr this might be better

test_var=`expr $am_i_numeric \* 0` >/dev/null 2>&1
if [ "$test_var" = "" ]
then
    ......

If numeric, multiplied by 1 gives you the same value, (including negative numbers). Otherwise you get null which you can test for

test -z "${i//[0-9]}" && echo digits || echo no no no

${i//[0-9]} replaces any digit in the value of $i with an empty string, see man -P 'less +/parameter\/' bash. -z checks if resulting string has zero length.

if you also want to exclude the case when $i is empty, you could use one of these constructions:

test -n "$i" && test -z "${i//[0-9]}" && echo digits || echo not a number
[[ -n "$i" && -z "${i//[0-9]}" ]] && echo digits || echo not a number

The simplest way is to check whether it contains non-digit characters. You replace all digit characters with nothing and check for length. If there's length it's not a number.

if [[ ! -n ${input//[0-9]/} ]]; then
    echo "Input Is A Number"
fi

Did the same thing here with a regular expression that test the entire part and decimals part, separated with a dot.

re="^[0-9]*[.]{0,1}[0-9]*$"

if [[ $1 =~ $re ]] 
then
   echo "is numeric"
else
  echo "Naahh, not numeric"
fi

I'm surprised at the solutions directly parsing number formats in shell. shell is not well suited to this, being a DSL for controlling files and processes. There are ample number parsers a little lower down, for example:

isdecimal() {
  # filter octal/hex/ord()
  num=$(printf '%s' "$1" | sed "s/^0*\([1-9]\)/\1/; s/'/^/")

  test "$num" && printf '%f' "$num" >/dev/null 2>&1
}

Change '%f' to whatever particular format you require.

This can be achieved by using grep to see if the variable in question matches an extended regular expression.

Test integer 1120:

yournumber=1120
if echo "$yournumber" | grep -qE '^[0-9]+$'; then
    echo "Valid number."
else
    echo "Error: not a number."
fi

Output: Valid number.

Test non-integer 1120a:

yournumber=1120a
if echo "$yournumber" | grep -qE '^[0-9]+$'; then
    echo "Valid number."
else
    echo "Error: not a number."
fi

Output: Error: not a number.

Explanation

• The grep, the -E switch allows us to use extended regular expression '^[0-9]+$'. This regular expression means the variable should only [] contain the numbers 0-9 zero through nine from the ^ beginning to the $ end of the variable and should have at least + one character.
• The grep, the -q quiet switch turns off any output whether or not it finds anything.
• if checks the exit status of grep. Exit status 0 means success and anything greater means an error. The grep command has an exit status of 0 if it finds a match and 1 when it doesn't;

So putting it all together, in the if test, we echo the variable $yournumber and | pipe it to grep which with the -q switch silently matches the -E extended regular expression '^[0-9]+$' expression. The exit status of grep will be 0 if grep successfully found a match and 1 if it didn't. If succeeded to match, we echo "Valid number.". If it failed to match, we echo "Error: not a number.".

For Floats or Doubles

We can just change the regular expression from '^[0-9]+$' to '^[0-9]*\.?[0-9]+$' for floats or doubles.

Test float 1120.01:

yournumber=1120.01
if echo "$yournumber" | grep -qE '^[0-9]*\.?[0-9]+$'; then
    echo "Valid number."
else
    echo "Error: not a number."
fi

Output: Valid number.

Test float 11.20.01:

yournumber=11.20.01
if echo "$yournumber" | grep -qE '^[0-9]*\.?[0-9]+$'; then
    echo "Valid number."
else
    echo "Error: not a number."
fi

Output: Error: not a number.

For Negatives

To allow negative integers, just change the regular expression from '^[0-9]+$' to '^\-?[0-9]+$'.

To allow negative floats or doubles, just change the regular expression from '^[0-9]*\.?[0-9]+$' to '^\-?[0-9]*\.?[0-9]+$'.

You could use "let" too like this :

[ ~]$ var=1
[ ~]$ let $var && echo "It's a number" || echo "It's not a number"
It\'s a number
[ ~]$ var=01
[ ~]$ let $var && echo "It's a number" || echo "It's not a number"
It\'s a number
[ ~]$ var=toto
[ ~]$ let $var && echo "It's a number" || echo "It's not a number"
It\'s not a number
[ ~]$ 

But I prefer use the "=~" Bash 3+ operator like some answers in this thread.

As i had to tamper with this lately and like karttu's appoach with the unit test the most. I revised the code and added some other solutions too, try it out yourself to see the results:

#!/bin/bash

    # N={0,1,2,3,...} by syntaxerror
function isNaturalNumber()
{
 [[ ${1} =~ ^[0-9]+$ ]]
}
    # Z={...,-2,-1,0,1,2,...} by karttu
function isInteger() 
{
 [[ ${1} == ?(-)+([0-9]) ]]
}
    # Q={...,-½,-¼,0.0,¼,½,...} by karttu
function isFloat() 
{
 [[ ${1} == ?(-)@(+([0-9]).*([0-9])|*([0-9]).+([0-9]))?(E?(-|+)+([0-9])) ]]
}
    # R={...,-1,-½,-¼,0.E+n,¼,½,1,...}
function isNumber()
{
 isNaturalNumber $1 || isInteger $1 || isFloat $1
}

bools=("TRUE" "FALSE")
int_values="0 123 -0 -123"
float_values="0.0 0. .0 -0.0 -0. -.0 \
    123.456 123. .456 -123.456 -123. -.456 \
    123.456E08 123.E08 .456E08 -123.456E08 -123.E08 -.456E08 \
    123.456E+08 123.E+08 .456E+08 -123.456E+08 -123.E+08 -.456E+08 \
    123.456E-08 123.E-08 .456E-08 -123.456E-08 -123.E-08 -.456E-08"
false_values="blah meh mooh blah5 67mooh a123bc"

for value in ${int_values} ${float_values} ${false_values}
do
    printf "  %5s=%-30s" $(isNaturalNumber $value) ${bools[$?]} $(printf "isNaturalNumber(%s)" $value)
    printf "%5s=%-24s" $(isInteger $value) ${bools[$?]} $(printf "isInteger(%s)" $value)
    printf "%5s=%-24s" $(isFloat $value) ${bools[$?]} $(printf "isFloat(%s)" $value)
    printf "%5s=%-24s\n" $(isNumber $value) ${bools[$?]} $(printf "isNumber(%s)" $value)
done

So isNumber() includes dashes, commas and exponential notation and therefore returns TRUE on integers & floats where on the other hand isFloat() returns FALSE on integer values and isInteger() likewise returns FALSE on floats. For your convenience all as one liners:

isNaturalNumber() { [[ ${1} =~ ^[0-9]+$ ]]; }
isInteger() { [[ ${1} == ?(-)+([0-9]) ]]; }
isFloat() { [[ ${1} == ?(-)@(+([0-9]).*([0-9])|*([0-9]).+([0-9]))?(E?(-|+)+([0-9])) ]]; }
isNumber() { isNaturalNumber $1 || isInteger $1 || isFloat $1; }

I use printf as other answers mentioned, if you supply the format string "%f" or "%i" printf will do the checking for you. Easier than reinventing the checks, the syntax is simple and short and printf is ubiquitous. So its a decent choice in my opinion - you can also use the following idea to check for a range of things, its not only useful for checking numbers.

declare  -r CHECK_FLOAT="%f"  
declare  -r CHECK_INTEGER="%i"  

 ## <arg 1> Number - Number to check  
 ## <arg 2> String - Number type to check  
 ## <arg 3> String - Error message  
function check_number() { 
  local NUMBER="${1}" 
  local NUMBER_TYPE="${2}" 
  local ERROR_MESG="${3}"
  local -i PASS=1 
  local -i FAIL=0   
  case "${NUMBER_TYPE}" in 
    "${CHECK_FLOAT}") 
        if ((! $(printf "${CHECK_FLOAT}" "${NUMBER}" &>/dev/random;echo $?))); then 
           echo "${PASS}"
        else 
           echo "${ERROR_MESG}" 1>&2
           echo "${FAIL}"
        fi 
        ;;                 
    "${CHECK_INTEGER}") 
        if ((! $(printf "${CHECK_INTEGER}" "${NUMBER}" &>/dev/random;echo $?))); then 
           echo "${PASS}"
        else 
           echo "${ERROR_MESG}" 1>&2
           echo "${FAIL}"
        fi 
        ;;                 
                     *) 
        echo "Invalid number type format: ${NUMBER_TYPE} to check_number()." 1>&2
        echo "${FAIL}"
        ;;                 
   esac
} 

>$ var=45

>$ (($(check_number $var "${CHECK_INTEGER}" "Error: Found $var - An integer is required."))) && { echo "$var+5" | bc; }

printf '%b' "-123\nABC" | tr '[:space:]' '_' | grep -q '^-\?[[:digit:]]\+$' && echo "Integer." || echo "NOT integer."

Remove the -\? in grep matching pattern if you don't accept negative integer.

The accepted answer does not work here, I am on MacOS. The following code works:

if [ $(echo "$number" | grep -c '^[0-9]\+$') = 0 ]; then 
    echo "it is a number"
else
    echo "not a number"
fi

To catch negative numbers:

if [[ $1 == ?(-)+([0-9.]) ]]
    then
    echo number
else
    echo not a number
fi

The accepted answer didn't work for me in all cases BASH 4+ so :

# -- is var an integer? --
# trim leading/trailing whitespace, then check for digits return 0 or 1
# Globals: None
# Arguments: string
# Returns: boolean
# --
is_int() {
    str="$(echo -e "${1}" | sed -e 's/^[[:space:]]*//' -e 's/[[:space:]]*$//')"
    case ${str} in ''|*[!0-9]*) return 1 ;; esac
    return 0
}

How to use it ?

Valid (will return 0 = true):

is_int "100" && echo "return 0" || echo "return 1"

Invalid (will return 1 = false) :

is_int "100abc" && echo "returned 0" || echo "returned 1"
is_int ""  && echo "returned 0" || echo "returned 1"
is_int "100 100"  && echo "returned 0" || echo "returned 1"
is_int "      "  && echo "returned 0" || echo "returned 1"
is_int $NOT_SET_VAR  && echo "returned 0" || echo "returned 1"
is_int "3.14"   && echo "returned 0" || echo "returned 1"

Output:

returned 0
returned 1
returned 1
returned 1
returned 1
returned 1
returned 1

note, in Bash, 1 = false, 0 = true. I am simply printing it out where instead something like this would be more likely :

if is_int ${total} ; then
    # perform some action 
fi

For my problem, I only needed to ensure that a user doesn't accidentally enter some text thus I tried to keep it simple and readable

isNumber() {
    (( $1 )) 2>/dev/null
}

According to the man page this pretty much does what I want

If the value of the expression is non-zero, the return status is 0

To prevent nasty error messages for strings that "might be numbers" I ignore the error output

$ (( 2s ))
bash: ((: 2s: value too great for base (error token is "2s")

I like Alberto Zaccagni's answer.

if [ "$var" -eq "$var" ] 2>/dev/null; then

Important prerequisites: - no subshells spawned - no RE parsers invoked - most shell applications don't use real numbers

But if $var is complex (e.g. an associative array access), and if the number will be a non-negative integer (most use-cases), then this is perhaps more efficient?

if [ "$var" -ge 0 ] 2> /dev/null; then ..

I use expr. It returns a non-zero if you try to add a zero to a non-numeric value:

if expr -- "$number" + 0 > /dev/null 2>&1
then
    echo "$number is a number"
else
    echo "$number isn't a number"
fi

It might be possible to use bc if you need non-integers, but I don't believe bc has quite the same behavior. Adding zero to a non-number gets you zero and it returns a value of zero too. Maybe you can combine bc and expr. Use bc to add zero to $number. If the answer is 0, then try expr to verify that $number isn't zero.

I use the following (for integers):

## ##### constants
##
## __TRUE - true (0)
## __FALSE - false (1)
##
typeset -r __TRUE=0
typeset -r __FALSE=1

## --------------------------------------
## isNumber
## check if a value is an integer 
## usage: isNumber testValue 
## returns: ${__TRUE} - testValue is a number else not
##
function isNumber {
  typeset TESTVAR="$(echo "$1" | sed 's/[0-9]*//g' )"
  [ "${TESTVAR}"x = ""x ] && return ${__TRUE} || return ${__FALSE}
}

isNumber $1 
if [ $? -eq ${__TRUE} ] ; then
  print "is a number"
fi

A clear answer has already been given by @charles Dufy and others. A pure bash solution would be using the following :

string="-12,345"
if [[ "$string" =~ ^-?[0-9]+[.,]?[0-9]*$ ]]
then
    echo $string is a number
else
    echo $string is not a number
fi

Although for real numbers it is not mandatory to have a number before the radix point.

To provide a more thorough support of floating numbers and scientific notation (many programs in C/Fortran or else will export float this way), a useful addition to this line would be the following :

string="1.2345E-67"
if [[ "$string" =~ ^-?[0-9]*[.,]?[0-9]*[eE]?-?[0-9]+$ ]]
then
    echo $string is a number
else
    echo $string is not a number
fi

Thus leading to a way to differentiate types of number, if you are looking for any specific type :

string="-12,345"
if [[ "$string" =~ ^-?[0-9]+$ ]]
then
    echo $string is an integer
elif [[ "$string" =~ ^-?[0-9]*[.,]?[0-9]*$ ]]
then
    echo $string is a float
elif [[ "$string" =~ ^-?[0-9]*[.,]?[0-9]*[eE]-?[0-9]+$ ]]
then
    echo $string is a scientific number
else
    echo $string is not a number
fi

Note: We could list the syntactical requirements for decimal and scientific notation, one being to allow comma as radix point, as well as ".". We would then assert that there must be only one such radix point. There can be two +/- signs in an [Ee] float. I have learned a few more rules from Aulu's work, and tested against bad strings such as '' '-' '-E-1' '0-0'. Here are my regex/substring/expr tools that seem to be holding up:

parse_num() {
 local r=`expr "$1" : '.*\([.,]\)' 2>/dev/null | tr -d '\n'` 
 nat='^[+-]?[0-9]+[.,]?$' \
 dot="${1%[.,]*}${r}${1##*[.,]}" \
 float='^[\+\-]?([.,0-9]+[Ee]?[-+]?|)[0-9]+$'
 [[ "$1" == $dot ]] && [[ "$1" =~ $float ]] || [[ "$1" =~ $nat ]]
} # usage: parse_num -123.456

I tried ultrasawblade's recipe as it seemed the most practical to me, and couldn't make it work. In the end i devised another way though, based as others in parameter substitution, this time with regex replacement:

[[ "${var//*([[:digit:]])}" ]]; && echo "$var is not numeric" || echo "$var is numeric"

It removes every :digit: class character in $var and checks if we are left with an empty string, meaning that the original was only numbers.

What i like about this one is its small footprint and flexibility. In this form it only works for non-delimited, base 10 integers, though surely you can use pattern matching to suit it to other needs.