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/nullthen    echo "$1 is an integer !!"else    echo "ERROR: first parameter must be an integer."    echo $USAGE    exit 1fi

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 123123 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 -123cERROR: first parameter must be an integer.$ int_check.sh 123cERROR: first parameter must be an integer.$ int_check.sh c123ERROR: 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:

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

Latecomer to the party here. I'm extremely surprised none of the answers mention the simplest, fastest, most portable solution; the case statement.

case ${variable#[-+]} in  *[!0-9]* | '') echo Not a number ;;  * ) echo Valid number ;;esac

The trimming of any sign before the comparison feels like a bit of a hack, but that makes the expression for the case statement so much simpler.