9.4. Typing variables: declare or typeset

declare/typeset options

-r readonly

declare -r var1

(declare -r var1 works the same as readonly var1)

This is the rough equivalent of the C const type qualifier. An attempt to change the value of a readonly variable fails with an error message.

-i integer

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

Certain arithmetic operations are permitted for declared integer variables without the need for expr or let.

n=6/3
echo "n = $n" # n = 6/3
declare -i n
n=6/3
echo "n = $n" # n = 2

-a array

declare -a indices

The variable indices will be treated as an array.

-f functions

declare -f

A declare -f line with no arguments in a script causes a listing of all the functions previously defined in that script.

declare -f function_name

A declare -f function_name in a script lists just the function named.

-x export

declare -x var3

This declares a variable as available for exporting outside the environment of the script itself.

-x var=$value

declare -x var3=373

The declare command permits assigning a value to a variable in the same statement as setting its properties.

Example 9-21. Using declare to type variables

#!/bin/bash
func1 ()
{
echo This is a function.
}
declare -f # Lists the function above.
echo
declare -i var1 # var1 is an integer.
var1=2367
echo "var1 declared as $var1"
var1=var1+1 # Integer declaration eliminates the need for 'let'.
echo "var1 incremented by 1 is $var1."
# Attempt to change variable declared as integer.
echo "Attempting to change var1 to floating point value, 2367.1."
var1=2367.1 # Results in error message, with no change to variable.
echo "var1 is still $var1"
echo
declare -r var2=13.36 # 'declare' permits setting a variable property
 #+ and simultaneously assigning it a value.
echo "var2 declared as $var2" # Attempt to change readonly variable.
var2=13.37 # Generates error message, and exit from script.
echo "var2 is still $var2" # This line will not execute.
exit 0 # Script will not exit here.