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Saturday, June 21, 2008

Posted by venu k
6 comments | 12:24 AM
Unlike many other programming languages, Bash does not separate 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.
Integer or string?
# Integer.
let "a += 1"
echo "a = $a "
# a = 2335
# Integer, still.

# 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 = $c"
# c = BB34
# Substitute "23" for "BB".
# This makes $d an integer
echo "d = $d"
# d = 2334
let "d += 1"
# 2334 + 1 =
echo "d = $d"
# d = 2335
# What about null variables?
echo "e = $e"
# e =
let "e += 1"
# Arithmetic operations allowed on a null variable?
echo "e = $e"
# e = 1
# 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
# Undeclared variable transformed into an integer.

The burden is on the programmer to keep track of what type the script variables are.
Bash will not do it for you.
But “declare” or “typeset “built-ins permit restring the properties of variables.
This is very weak form of the typing available in certain programming languages

declare –i number
# The script will treat subsequent occurrences of “number” as an integer
echo “Number = $number”
# Number = 3
echo “Number = $number”
#Number = 0
#Tries to evaluate the string “three” as an integer