9.2. Typing variables: declare or typeset
The declare or typeset builtins, which are exact synonyms, permit modifying the properties of variables. This is a very weak form of the typing [1] available in certain programming languages. The declare command is specific to version 2 or later of Bash. The typeset command also works in ksh scripts.
- -r readonly
(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.
declare -r var1=1 echo "var1 = $var1" # var1 = 1 (( var1++ )) # x.sh: line 4: var1: readonly variable
- -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 = $n" # n = 2
- -a array
declare -a indices
The variable indices will be treated as an array.
- -f function(s)
declare -f
A declare -f line with no arguments in a script causes a listing of all the functions previously defined in that script.
declare -f function_name
A declare -f function_name in a script lists just the function named.
- -x export
declare -x var3
This declares a variable as available for exporting outside the environment of the script itself.
- -x var=$value
declare -x var3=373
The declare command permits assigning a value to a variable in the same statement as setting its properties.
Example 9-10. 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 $var2" # This line will not execute.
exit 0 # Script will not exit here. |
 | Using the declare builtin restricts the scope of a variable.
However . . .
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9.2.1. Another use for declare
The declare command can be helpful in identifying variables, environmental or otherwise. This can be especially useful with arrays.
bash$ declare | grep HOME
HOME=/home/bozo
bash$ zzy=68
bash$ declare | grep zzy
zzy=68
bash$ Colors=([0]="purple" [1]="reddish-orange" [2]="light green")
bash$ echo ${Colors[@]}
purple reddish-orange light green
bash$ declare | grep Colors
Colors=([0]="purple" [1]="reddish-orange" [2]="light green")
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