The Bash declare Statement

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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").

______________________

Mitch Frazier is an Associate Editor for Linux Journal.

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RE: "All variables can be used as arrays without explicit..."

Mark Thompson's picture

"All variables can be used as arrays without explicit definition" is not always true, at least in bash ~3.0. If you define a variable using "local" in a function, this is apparently nearly, if not exactly, the same as using "declare", which as mentioned does define a variable as local when used within a function. If when defining a variable as local you do not also use "-a", then you cannot also use the array initialization notation to treat it as an array; you end up with all initialization values in the first element of the "array" you just created. (Using "declare" unstead of "local" seemed to work just the same in my limited testing.) If you just remove the "local" declaration, your intended array works just fine, but it is now global.

Its nice to know that bash

rahulms's picture

Its nice to know that bash declare statement can be used to declare variable to accept only numeric values.

a=13 declare -r a echo

dlbb's picture

a=13
declare -r a
echo $a
a=14
echo
I can not execute this command on my pc, anyone help me?

Are you Using Windows?

Mitch Frazier's picture

It won't work if you are.

Perhaps if you leave a few more details...

Mitch Frazier is an Associate Editor for Linux Journal.

is linux so great??

Joyce Johnson's picture

I had the opportunity to use the linux redhat stuff about 5 years ago. It had the same amount of bugs as the windows competitor at the time. I don't understand the big hooplah. Instead of blue-screening, the PC would just shut down.

re: bash

TMJ's picture

I runned #!/bin/bash

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

but it fail , did i miss anything ? i think i did it right with commands..

Nope You Didn't Miss Anything

Mitch Frazier's picture

It's *supposed* to fail, read the paragraph after the script:

> When run, the second assignment will fail:
>
> $ sh decl.sh
> 13
> decl.sh: line 6: a: readonly variable

Mitch Frazier is an Associate Editor for Linux Journal.

Is there any difference

Anonymous's picture

Is there any difference between using 'declare -r' and 'readonly' to set variables as read only?

No Difference

Mitch Frazier's picture

Although the man page does not explicitly say they are the same, there does not appear to be any difference in the effect that they have or the error message that's produced if you try to change one.

Mitch Frazier is an Associate Editor for Linux Journal.

Exact same

Omer's picture

It doesn't explain it anywhere on the page, but yeah they're the same for all practical purposes.

thanks for clarity

Natural Stone's picture

I wasn't sure about that either thanks for clearing my confusion.

You can do these with

Anonymous's picture

You can do these with typeset too and it's more portable.

to what?

xtifr's picture

More portable to what? "help typeset" says "Obsolete. See `declare'." And there's no mention of "typeset" in Posix. I was under the impression that it was a korn-shell-ism, and ksh makes little or no pretense to being a viable substiture for a standard /bin/sh, so I see no reason to worry about compatibility with it. Dash (another widely used bourne/posix shell, based on BSD's ash) has "readonly" but neither declare nor typeset, so for that particular case, I would think that "readonly" (also supported by ksh, fwiw) would be slightly more portable. Aside from that, the only factor I see for choosing between the two is that "typeset" looks like something that may not be supported for much longer.

I didn't say typeset is a

Anonymous's picture

I didn't say typeset is a POSIX standard, but works with more shells, like you said ksh -- and its variants. Well, bash is the default shell on Linux, but ksh is very popular on other Unix-like systems.

The fact that typeset is obsolete in bash is surprised me. I can't see why is it good to removing typeset from bash.

help

xtifr's picture

Actually, typing "help declare" at the prompt may be quicker/easier than hunting through the very-large man page, at least to get started. (Many people don't realize that bash has built-in help for its built-in commands.)

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