| Advanced Bash-Scripting Guide: An in-depth exploration of the art of shell scripting | ||
|---|---|---|
| Prev | Chapter 4. Introduction to Variables and Parameters | Next |
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?
1 #!/bin/bash
2 # int-or-string.sh
3
4 a=2334 # Integer.
5 let "a += 1"
6 echo "a = $a " # a = 2335
7 echo # Integer, still.
8
9
10 b=${a/23/BB} # Substitute "BB" for "23".
11 # This transforms $b into a string.
12 echo "b = $b" # b = BB35
13 declare -i b # Declaring it an integer doesn't help.
14 echo "b = $b" # b = BB35
15
16 let "b += 1" # BB35 + 1 =
17 echo "b = $b" # b = 1
18 echo
19
20 c=BB34
21 echo "c = $c" # c = BB34
22 d=${c/BB/23} # Substitute "23" for "BB".
23 # This makes $d an integer.
24 echo "d = $d" # d = 2334
25 let "d += 1" # 2334 + 1 =
26 echo "d = $d" # d = 2335
27 echo
28
29 # What about null variables?
30 e=""
31 echo "e = $e" # e =
32 let "e += 1" # Arithmetic operations allowed on a null variable?
33 echo "e = $e" # e = 1
34 echo # Null variable transformed into an integer.
35
36 # What about undeclared variables?
37 echo "f = $f" # f =
38 let "f += 1" # Arithmetic operations allowed?
39 echo "f = $f" # f = 1
40 echo # Undeclared variable transformed into an integer.
41
42
43
44 # Variables in Bash are essentially untyped.
45
46 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.