The Bash declare Statement
April 21st, 2009 by Mitch Frazier in
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 and the Web Editor for linuxjournal.com.
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a=13 declare -r a echo
On June 10th, 2009 dlbb (not verified) says:
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?
On June 10th, 2009 Mitch Frazier says:
It won't work if you are..png)
Perhaps if you leave a few more details...
__________________________Mitch Frazier is an Associate Editor for Linux Journal and the Web Editor for linuxjournal.com.
is linux so great??
On July 27th, 2009 Joyce Johnson (not verified) says:
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
On June 9th, 2009 TMJ (not verified) says:
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
On June 9th, 2009 Mitch Frazier says:
It's *supposed* to fail, read the paragraph after the script:
__________________________Mitch Frazier is an Associate Editor for Linux Journal and the Web Editor for linuxjournal.com.
Is there any difference
On April 29th, 2009 Anonymous (not verified) says:
Is there any difference between using 'declare -r' and 'readonly' to set variables as read only?
No Difference
On April 30th, 2009 Mitch Frazier says:
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 and the Web Editor for linuxjournal.com.
Exact same
On June 15th, 2009 Omer (not verified) says:
It doesn't explain it anywhere on the page, but yeah they're the same for all practical purposes.
thanks for clarity
On June 11th, 2009 Natural Stone (not verified) says:
I wasn't sure about that either thanks for clearing my confusion.
You can do these with
On April 22nd, 2009 Anonymous (not verified) says:
You can do these with typeset too and it's more portable.
to what?
On April 22nd, 2009 xtifr says:
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
On April 22nd, 2009 Anonymous (not verified) says:
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
On April 21st, 2009 xtifr says:
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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