4.3. 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.

Example 4-4. Integer or string?

#!/bin/bash # int-or-string.sh a=2334 # Integer. let "a += 1" echo "a = $a " # a = 2335 echo # Integer, still. b=${a/23/BB} # Substitute "BB" for "23". # This transforms $b into a string. echo "b = $b" # b = BB35 declare -i b # Declaring it an integer doesn't help. echo "b = $b" # b = BB35 let "b += 1" # BB35 + 1 = echo "b = $b" # b = 1 echo c=BB34 echo "c = $c" # c = BB34 d=${c/BB/23} # Substitute "23" for "BB". # This makes $d an integer. echo "d = $d" # d = 2334 let "d += 1" # 2334 + 1 = echo "d = $d" # d = 2335 echo # What about null variables? e="" echo "e = $e" # e = let "e += 1" # Arithmetic operations allowed on a null variable? echo "e = $e" # e = 1 echo # Null variable transformed into an integer. # What about undeclared variables? echo "f = $f" # f = let "f += 1" # Arithmetic operations allowed? echo "f = $f" # f = 1 echo # Undeclared variable transformed into an integer. # Variables in Bash are essentially untyped. exit 0

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

To lighten the burden of keeping track of variable types in a script, Bash does permit declaring variables.