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.

declare/typeset options

-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 9-23. 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.

Caution

Using the declare builtin restricts the scope of a variable.
foo () { FOO="bar" } bar () { foo echo $FOO } bar # Prints bar.

However . . .
foo (){ declare FOO="bar" } bar () { foo echo $FOO } bar # Prints nothing. # Thank you, Michael Iatrou, for pointing this out.

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.

declare -i intvar intvar=23 echo "$intvar" # 23 intvar=stringval echo "$intvar" # 0