Test whether string is a valid integer

[[ $var =~ ^-?[0-9]+$ ]]

• The ^ indicates the beginning of the input pattern
• The - is a literal "-"
• The ? means "0 or 1 of the preceding (-)"
• The + means "1 or more of the preceding ([0-9])"
• The $ indicates the end of the input pattern

So the regex matches an optional - (for the case of negative numbers), followed by one or more decimal digits.

References:

• http://www.tldp.org/LDP/abs/html/bashver3.html#REGEXMATCHREF

Wow... there are so many good solutions here!! Of all the solutions above, I agree with @nortally that using the -eq one liner is the coolest.

I am running GNU bash, version 4.1.5 (Debian). I have also checked this on ksh (SunSO 5.10).

Here is my version of checking if $1 is an integer or not:

if [ "$1" -eq "$1" ] 2>/dev/null
then
    echo "$1 is an integer !!"
else
    echo "ERROR: first parameter must be an integer."
    echo $USAGE
    exit 1
fi

This approach also accounts for negative numbers, which some of the other solutions will have a faulty negative result, and it will allow a prefix of "+" (e.g. +30) which obviously is an integer.

Results:

$ int_check.sh 123
123 is an integer !!

$ int_check.sh 123+
ERROR: first parameter must be an integer.

$ int_check.sh -123
-123 is an integer !!

$ int_check.sh +30
+30 is an integer !!

$ int_check.sh -123c
ERROR: first parameter must be an integer.

$ int_check.sh 123c
ERROR: first parameter must be an integer.

$ int_check.sh c123
ERROR: first parameter must be an integer.

The solution provided by Ignacio Vazquez-Abrams was also very neat (if you like regex) after it was explained. However, it does not handle positive numbers with the + prefix, but it can easily be fixed as below:

[[ $var =~ ^[-+]?[0-9]+$ ]]