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9.4. 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 var1works the same asreadonly 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 = $n" # n = 2
- -a
array declare -a indices
The variable
indiceswill be treated as an array.- -f
function(s) declare -f
A
declare -fline with no arguments in a script causes a listing of all the functions previously defined in that script.declare -f function_name
A
declare -f function_namein a script lists just the function named.- -x export
declare -x var3
This declares a variable as available for exporting outside the environment of the script itself.
- -x var=$value
declare -x var3=373
The declare command permits assigning a value to a variable in the same statement as setting its properties.
Example 9-23. 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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9.4.1. Another use for declare
The declare command can be helpful in identifying variables, environmental or otherwise. This can be especially useful with arrays.
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Notes
| [1] | In this context, typing a variable means to classify it and restrict its properties. For example, a variable declared or typed as an integer is no longer available for string operations.
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