The Bash declare Statement

Although rarely used, the bash declare statement does have a couple useful options. It can mark a variable as read only and also mark it as being a number only.

To declare a variable as read only, use the following statement:

declare -r varname

Consider the following script:

#!/bin/bash

a=13
declare -r a
echo $a
a=14
echo $a

When run, the second assignment will fail:

$ sh decl.sh
13
decl.sh: line 6: a: readonly variable

To declare that a variable should accept only numeric values (integers), use the following statement:

declare -i varname

Consider the following script:

#!/bin/bash

declare -i a
a=12
echo $a
a=hello
echo $a

When run, the second assignment will assign zero to the variable rather than the string "hello" that appears in the statement:

$ sh decl2.sh
12
0

The declare statment has other options; the -a option can be used to declare a variable as an array, but it's not necessary. All variables can be used as arrays without explicit definition. As a matter of fact, it appears that in a sense, all variables are arrays, and that assignment without a subscript is the same as assigning to "[0]". Consider the following script:

#!/bin/bash

a=12
echo ${a[0]}
b[0]=13
echo $b

When run it produces:

$ sh arr.sh
12
13

For further options, see the bash man page (search for "^SHELL BUILTINS", then search for "declare").