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: Integer or string?
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 (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.
