I'm trying to check if an input is an integer and I've gone over it a hundred times but don't see the error in this. Alas it does not work, it triggers the if statement for all inputs (numbers/letters)
read scale
if ! [[ "$scale" =~ "^[0-9]+$" ]]
then
echo "Sorry integers only"
fi
I've played around with the quotes but either missed it or it did nothing. What do I do wrong? Is there an easier way to test if an input is just an INTEGER?
Use -eq operator of test command:
read scale
if ! [ "$scale" -eq "$scale" ] 2> /dev/null
then
echo "Sorry integers only"
fi
It not only works in bash but also any POSIX shell. From POSIX test documentation:
n1 -eq n2
True if the integers n1 and n2 are algebraically equal; otherwise, false.
[[ instead of old test [ as mine.
– cuonglm
Aug 22 '14 at 17:27
declare -i scale before the read command). The problem occurs during assignment to an integer variable because any string will be interpreted as a variable name, and dereferenced (recursively). Unset or null "variable names" evaluate to 0. E.g. unset abc; declare -i a; a=abc; # a = 0 or a=3; b=a; c=b; declare -i d=c; # d=3.
– SpinUp __ A Davis
Mar 22 '24 at 17:49
As the OP seems to want only positive integers:
[ "$1" -ge 0 ] 2>/dev/null
Examples:
$ is_positive_int(){ [ "$1" -ge 0 ] 2>/dev/null && echo YES || echo no; }
$ is_positive_int word
no
$ is_positive_int 2.1
no
$ is_positive_int -3
no
$ is_positive_int 42
YES
Note that a single [ test is required:
$ [[ "word" -eq 0 ]] && echo word equals zero || echo nope
word equals zero
$ [ "word" -eq 0 ] && echo word equals zero || echo nope
-bash: [: word: integer expression expected
nope
This is because dereferencing occurs with [[:
$ word=other
$ other=3
$ [[ $word -eq 3 ]] && echo word equals other equals 3
word equals other equals 3
" 123 " for instance. You wouldn't want to use that approach in ksh where it would not only work but also be a command injection vulnerability.
– Stéphane Chazelas
Jan 25 '21 at 10:50
A POSIX and portable solution is:
read scale
if [ -z "${scale##*[!0-9]*}" ];
then echo "Sorry integers only"
fi
scale=1-2. how to handle such a case depends on the application.
– SpinUp __ A Davis
Mar 22 '24 at 14:50
For unsigned integers I use:
read -r scale
[ -z "${scale//[0-9]}" ] && [ -n "$scale" ] || echo "Sorry integers only"
Tests:
$ ./test.sh
7
$ ./test.sh
777
$ ./test.sh
a
Sorry integers only
$ ./test.sh
""
Sorry integers only
$ ./test.sh
Sorry integers only
( scale=${scale##*[!0-9]*}
: ${scale:?input must be an integer}
) || exit
That does the check and outputs your error.
$@: params=("${@##*[!0-9]*}") -- Keep all positional arguments, but only use numbers - anything else will be converted to null string which conveniently evaluates to 0 in arithmetic evaluations (my use case - function parameter is a list of integers, unset or null should default to 0 but caller sets variable name when unset).
– Thomas Guyot-Sionnest
Apr 21 '20 at 22:10
POSIXly:
case $1 in
("" | *[!0123456789]*)
echo >&2 Sorry, decimal integer only
exit 1
esac
Do not use [0-9] which often matches a lot more than [0123456789] (especially with bash globs, YMMV for bash's [[ =~ ]] operator which uses the system regexps where [0-9] may or may not match more than 0123456789 depending on the system and locale).
More on that at what is the meaning of this shell script function
Note that you may also want to reject numbers like 08 or 09 which some tools (including bash arithmetic operators) reject as invalid octal numbers.
In bash, you can use the extended patterns inside [[...]]:
scale=4
[[ $scale == +([[:digit:]]) ]] || echo "Sorry integers only"
# ==> no output
scale=4.0
[[ $scale == +([[:digit:]]) ]] || echo "Sorry integers only"
# ==> Sorry integers only
For X greater than 0 I use
[ 0$X -gt 0 2>/dev/null ] && echo "is number" || echo "is string"
this gives no error if X is empty
X='x -o -v xx[$(reboot)1] -o 1' in bash (gives true and reboots).
– Stéphane Chazelas
Jan 25 '21 at 10:45
The negation is now in the regular expression instead of the test condition.
if [[ $scale =~ [^0-9] ]]
then
echo "Sorry integers only"
fi
Explanation:
=~ binary operator where the string to the right of the operator is considered an extended regular expression and matched accordingly (as in regex(3))[^0-9] matches a single character not present in the list 0-9.
[0-9] matches more than the 0123456789 characters.
– Stéphane Chazelas
Jan 08 '23 at 11:38
The following is POSIX compliant, so it will work in other shells than bash as well. Just checking -z "${scale##*[!0-9]*}" as suggested by another answer here does not take into account that strings starting with zero (except for zero itself) or empty strings are also not valid base-10 integers.
case "$scale" in *[!0123456789]*|0?*|"")
echo "Sorry integers only";;
esac
We need to spell out all valid digits because using a range like 0-9 matches on any character (or possibly multi-character collation element) that sorts in between 0 and 9. Thanks to @stéphane-chazelas for this insight. More about that here: https://lists.gnu.org/archive/html/bug-bash/2021-02/msg00054.html
If negative integers are valid in your use case, you can use:
case "$scale" in *[!0123456789-]*|-|?*-*|0?*|-0|"")
echo "Sorry integers only";;
esac
if [[ $scale =~ [^0-9] ]]is less convoluted. Doing so, puts the negation in the regular expression instead of the test condition. See below. – Serge Stroobandt Apr 14 '22 at 16:04