9.4. Typing variables: declare or typeset

The declare or typeset builtins (they are exact synonyms) permit restricting the properties of variables. This is a very weak form of the typing 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

   1 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

   1 declare -i number
   2 # The script will treat subsequent occurrences of "number" as an integer.		
   4 number=3
   5 echo "number = $number"     # number = 3
   7 number=three
   8 echo "number = $number"     # number = 0
   9 # Tries to evaluate "three" as an integer.
Note that certain arithmetic operations are permitted for declared integer variables without the need for expr or let.

-a array

   1 declare -a indices

The variable indices will be treated as an array.

-f functions

   1 declare -f

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

   1 declare -f function_name

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

-x export

   1 declare -x var3

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


   1 declare -x var3=373

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

Example 9-20. Using declare to type variables

   1 #!/bin/bash
   3 func1 ()
   4 {
   5 echo This is a function.
   6 }
   8 declare -f        # Lists the function above.
  10 echo
  12 declare -i var1   # var1 is an integer.
  13 var1=2367
  14 echo "var1 declared as $var1"
  15 var1=var1+1       # Integer declaration eliminates the need for 'let'.
  16 echo "var1 incremented by 1 is $var1."
  17 # Attempt to change variable declared as integer
  18 echo "Attempting to change var1 to floating point value, 2367.1."
  19 var1=2367.1       # Results in error message, with no change to variable.
  20 echo "var1 is still $var1"
  22 echo
  24 declare -r var2=13.36         # 'declare' permits setting a variable property
  25                               #+ and simultaneously assigning it a value.
  26 echo "var2 declared as $var2" # Attempt to change readonly variable.
  27 var2=13.37                    # Generates error message, and exit from script.
  29 echo "var2 is still $var2"    # This line will not execute.
  31 exit 0                        # Script will not exit here.