5.3. Bash Variables Are Untyped

Unlike many other programming languages, Bash does not segregate its variables by "type". Essentially, Bash variables are character strings, but, depending on context, Bash permits integer operations and comparisons on variables. The determining factor is whether the value of a variable contains only digits.

#!/bin/bash
# int-or-string.sh
# Integer or string?

a=2334                   # Integer.
let "a += 1"
echo "a = $a "           # Integer, still.
echo

b=${a/23/BB}             # Transform into a string.
echo "b = $b"            # BB35
declare -i b             # Declaring it an integer doesn't help.
echo "b = $b"            # BB35, still.

let "b += 1"             # BB35 + 1 =
echo "b = $b"            # 1
echo

c=BB34
echo "c = $c"            # BB34
d=${c/BB/23}             # Transform into an integer.
echo "d = $d"            # 2334
let "d += 1"             # 2334 + 1 =
echo "d = $d"            # 2335

# Variables in Bash are essentially untyped.

exit 0

Untyped variables are both a blessing and a curse. They permit more flexibility in scripting (enough rope to hang yourself) and make it easier to grind out lines of code. However, they permit errors to creep in and encourage sloppy programming habits.

The burden is on the programmer to keep track of what type the script variables are. Bash will not do it for you.