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

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

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

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

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

...or, to handle numbers with a sign:

^[+-]?[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 using non-numbers will result in an error which will be implicitly considered as false in shell:

"$var" -eq "$var"

as in:

#!/bin/bashvar=aif [ -n "$var" ] && [ "$var" -eq "$var" ] 2>/dev/null; then  echo numberelse  echo not a numberfi

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

[ -n "$var" ] && [ "$var" -eq "$var" ] 2>/dev/nullif [ $? -ne 0 ]; then   echo $var is not numberfi

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")