Define a set of values for a variable in shell script

Question

I want to define a set of values for a single variable. E.g.

singleDigit={0,1,2,3,4,5,6,7,8,9}

If the user gives 10 it should exit with an error.

How can this be achieved without using a loop, but an array instead?

Answer 1

If you want to store several values into a variable, that's where you'd use an array like:

allowed_values=(0 1 2 3 4 5 6 7 8 9)

Or

allowed_values=({0..9})

with bash3+ (syntax borrowed from zsh now also available in ksh93).

And if you want to check whether a given string is among any of those values, you could define a function like:

isin() {
  local s
  for s in "${@:2}"; do
    [[ $1 = "$s" ]] && return 0
  done
  return 1
}

if isin "$input" "${allowed_values[@]}"

(in zsh, you can do if ((allowed_values[(eI)$input]))).

Note that the comparison is lexical (as in 01 != 1). Do do numerical comparison, you'd use [ "$1" -eq "$s" ] 2> /dev/null (note that 010 is 10, not 8). Do not use [[ $1 -eq $s ]] or ((s == $1)) as those introduce arbitrary command injection vulnerabilities.

---

If you want to assign a pattern to a variable to match against some input, you'd use a scalar variable and use one of the 2 (3 with the extglob option) pattern syntaxes supported by bash:

• wildcard/glob pattern

  pattern='[0-9]'
  

  Used as:

  case $input in
    ($pattern) echo OK;;
    (*) echo NOK
  esac
  

  or

  if [[ $input = $pattern ]]; then
    echo OK
  else
    echo NOK
  fi
  

• Extended Regular Expression:

  regex='^[0-9]$'
  

  used as:

  if [[ $input =~ $regex ]]; then
    echo OK
  else
    echo NOK
  fi
  

In the general case, for values other than single-character ones, you'd use:

regex='^(one|two|three)$'

And for patterns, you'd need the extglob option:

shopt -s extglob
pattern='@(one|two|three)'

---

Another approach with bash4+, or ksh93 or zsh is to use an associative array:

With bash, ksh93:

typeset -A allowed_values
allowed_values=([one]=1 [two]=1 [three]=1)
if ((allowed_values[$input])); then
  echo OK
else
  echo NOK
fi

In zsh, same but the assignment syntax is:

typeset -A allowed_values
allowed_values=(one 1 two 1 three 1)

(note that with bash, you can't have the empty string as an allowed value).

Answer 2

If only the numbers 0 through 9 are allowable as input, all you need is a simple IF statement.

#!/bin/bash

echo -n Please enter a single digit:
read single_digit
if [ $single_digit -lt 10 ]; then
   echo Thanks for $single_digit
else
   echo Screwball!!! I said SINGLE DIGIT!!
fi