How to check if variable is integer (avoid problem with spaces around) in POSIX shell script?

Question

My POSIX is_integer () function looks like this for a long time:

#!/bin/sh
is_integer ()
{
 [ "1ドル" -eq "1ドル" ] 2> /dev/null
}

However, today, I found it broken. If there are some spaces around the number, it surprisingly also evaluates to true, and I have no idea how to fix that.

---

Example of correct (expected) behavior:

is_integer 123 evaluates to true.

Example of incorrect (unexpected) behavior:

is_integer ' 123' also evaluates to true, however it obviously contains a leading space, thus the function is expected to evaluate to false in such cases.

---

POSIX-compliant suggestions only, please. Thank you.

• ilkkachu: Note that even in the absence of spaces, you couldn't use that to detect integers in ksh, since extends the interpretation of the arguments of -eq quite a lot wider than to just integers. Stuff like abc (the value on variable abc), 12.345 (floating point), 1+1 (arithmetic expression) get accepted.

Answer 1

#!/bin/sh
is_integer ()
{
 case "${1#[+-]}" in
 (*[!0123456789]*) return 1 ;;
 ('') return 1 ;;
 (*) return 0 ;;
 esac
}

Uses only POSIX builtins. It is not clear from the spec if +1 is supposed to be an integer, if not then remove the + from the case line.

It works as follows. the ${1#[+-]} removes the optional leading sign. If you are left with something containing a non digit then it is not an integer, likewise if you are left with nothing. If it is not not an integer then it is an integer.

Edit: change ^ to ! to negate the character class - thanks @LinuxSecurityFreak

• Quasímodo: I don't get why ^ was changed to !. Isn't ^ used to negate the class? See RE bracket expressions
• Quasímodo: True! Just for the record, here the exception is documented in Shell Command Language, Pattern Matching Notation.
• icarus: Your tests should have some multi-digit strings in them. You might want to extend the patterns to reject leading zeros (accept '0', reject '0'*).
• icarus: @fpmurphy That is true that you can use [:digit:] but I would rather not have 123๔ as an integer because the last character is a digit 4 in Thai. In the original version of this post I use [^0-9] but changed it to explicitly list the characters I wanted to use in the definition of an integer.
• Stéphane Chazelas: @fpmurphy, on some BSDs, [[:digit:]] will match all decimal digits, not just the Arabic / ASCII ones, even in US English locales.

Answer 2

Not the most efficient (due to the external command), but quite simple:

is_integer () {
 expr "X1ドル" : "X-\{0,1\}[0-9][0-9]*$" > /dev/null
}

At least in the implementation I am testing, an initial argument - is treated not as part of a matching operation, but apparently as part of an invalid arithmetic expression; the X ensures expr parses its arguments as a valid match operation.

• Stéphane Chazelas: Don't use [0-9] for input validation, it often matches thousands of characters some of which can cause nasty problems if not filtered out. Use [0123456789] instead.

Answer 3

A more complete solution would be as follows:

is_integer() (
 export LC_ALL=C
 local n=${1#[-+]}
 case "$n" in
 0[0-7]*) case "$n" in 0*[!0-7]*) return 1;; esac;;
 0[xX]*) case "$n" in 0[xX]|0[xX]*[!0-9a-fA-F]*) return 1;; esac;;
 *) case "$n" in ''|*[!0-9]*) return 1;; esac;;
 esac
)

This strips any leading sign and then parses the string depending upon rather or not it has a prefix of 0, 0x or 0X. Thus, one should be aware not to have arbitrary leading zeros on a value that will be used as decimal number.

$ echo $((01))
1
$ echo $((08))
-ash: arithmetic syntax error

• Stéphane Chazelas: Don't use ranges like [0-7] for input validation! They often match thousands of characters some of which can cause nasty problems if not ruled out. Use [01234567]. Note that local is not POSIX.
• Stéphane Chazelas: Octal and hexadecimal are allowed in POSIX sh arithmetic expressions or in POSIX printf %d arguments, not in ['s -eq operand, but then again leading blanks are allowed in most of those as well. The OP didn't specify where the numbers were going to be used after being sanitised.
• Stéphane Chazelas: See for instance How to ensure user input consists of exactly 6 digits or Rename special characters in filenames to underscore or regex pattern issue for digit validation in ksh for instance here.
• FWDekker: @StéphaneChazelas Those aren't sources, those are links to you making similar comments. I've tried to reproduce the incorrect matching, but of the ~1000 characters that would supposedly match incorrectly, I cannot get a single match in any shell or regex implementation I've tried. Do you have a specific reproducible example in which the matching goes wrong?
• Stéphane Chazelas: @FWDekker try for instance LC_ALL=en_US.UTF-8 bash -c '[[ ۸ = [0-9] ]] && echo yes' on Ubuntu 22.04 for instance (same with the 1000+ other characters mentioned in those other questions). YMMV with the tool, libc, OSes and versions thereof.