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====== The declare builtin command ======
===== Synopsis =====
&lt;code&gt;
declare [-aAfFgilrtux] [-p] [NAME[=VALUE] ...]
# obsolete typeset synonym
typeset [-aAfFgilrtux] [-p] [NAME[=VALUE] ...]
&lt;/code&gt;
===== Description =====
''declare'' is used to display or set variables along with variable attributes. When used to display variables/functions and their value, the output is re-usable as input for the shell.
If no ''NAME'' is given, it displays the values of all variables or functions when restricted by the ''-f'' option.
If ''NAME'' is followed by ''=VALUE'', ''declare'' also sets the value for a variable.
When used in a function, ''declare'' makes ''NAMEs'' local variables, unless used with the ''-g'' option.
Don't use it's synonym ''typeset'' when coding for Bash, since it's tagged as obsolete.
==== Options ====
Below, ''[-+]X'' indicates an attribute, use ''-X'' to set the attribute, ''+X'' to remove it.
^Option ^Description ^
|''[-+]a'' |make NAMEs indexed arrays (removing with ''+a'' is valid syntax, but leads to an error message) |
|''[-+]A'' |make NAMEs associative arrays |
|''[-+]c'' | **Undocumented** convert NAMEs to &quot;capcase&quot; on assignment (makes the first letter upper-case and the rest lower). Requires Bash built with ''-DCASEMOD_CAPCASE'' |
|''-f'' |restrict action or display to function names and definitions (removing with ''+f'' is valid syntax, but leads to an error message) |
|''-F'' |restrict display to function names only (plus line number and source file when debugging) |
|''-g'' |create global variables when used in a shell function; otherwise ignored (by default, ''declare'' declares local scope variables when used in shell functions) |
|''[-+]i'' |make NAMEs have the &quot;integer&quot; attribute |
|''[-+]l'' |convert NAMEs to lower case on assignment (makes sure the variable contains only lower case letters) |
|''[-+]n'' |make NAME a reference to the variable named by its value. Introduced in Bash 4.3-alpha |
|''-p'' |display the attributes and value of each NAME |
|''[-+]r'' |make NAMEs readonly (removing with ''+r'' is valid syntax, but not possible) |
|''[-+]t'' |make NAMEs have the &quot;trace&quot; attribute (effective only for functions) |
|''[-+]u'' |convert NAMEs to upper case on assignment (makes sure the variable contains only upper case letters) |
|''[-+]x'' |make NAMEs exported |
==== Return status ====
^Status ^Reason ^
|0 |no error |
|!= 0 |invalid option |
|!= 0 |invalid variable name given |
|!= 0 |attempt to **define** a function using ''-f'' |
|!= 0 |assignment to a readonly variable |
|!= 0 |removing the readonly-attribute from a readonly variable |
|!= 0 |assignment to an array variable without the compound assignment syntax (''array=(...)'') |
|!= 0 |attempt to use ''+a'' to &quot;destroy&quot; an array |
|!= 0 |attemt to display a non-existent function with ''-f'' |
===== Notes =====
Unix shells offer very few datatypes. Bash and some other shells extend this by allowing &quot;attributes&quot; to be set on variable names. The only attributes specified by POSIX are ''export'' and ''readonly'', which are set by their own dedicated builtins. Datatypes in bash have a few other interesting capabilities such as the ability to modify data on assignment.
===== Examples =====
==== Display defined functions ====
''declare -f'' can be used to display all defined functions...
&lt;code&gt;
$ declare -f
foo ()
{
echo &quot;FOO is BAR&quot;
}
world ()
{
echo &quot;Hello World!&quot;
}
&lt;/code&gt;
...or just a specific defined function.
&lt;code&gt;
$ declare -f foo
foo ()
{
echo &quot;FOO is BAR&quot;
}
&lt;/code&gt;
==== Nameref ====
Bash 4.3 adds a new way to indirectly reference variables. ''typeset -n'' can be used to make a variable indirectly refer to another. In Bash, the lvalue of the assignment given to ''typeset -n'' will refer to the variable whose name is expanded on the RHS.
&lt;code&gt;
# Sum a set of arrays and assign the result indirectly, also printing each intermediary result (without portability workarounds)
# sum name arrname [ arrname ... ]
function sum {
typeset -n _result=$1 _arr
typeset IFS=+
_result=0
for _arr in &quot;${@:2}&quot;; do # Demonstrate the special property of &quot;for&quot; on a nameref.
(( _result += ${_arr[*]} ))
printf '%s = %d\n' &quot;${!_result}&quot; &quot;$_result&quot; # Demonstrate the special property of ${!ref} on a nameref.
done
}
a=(1 2 3) b=(6 5 4) c=(2 4 6)
sum total a b c
printf 'Final value of &quot;total&quot; is: %d\n' &quot;$total&quot;
&lt;/code&gt;
&lt;div hide&gt;
function sum {
typeset -n _result=$1
shift
typeset IFS=+ _arrx
_result=0
for _arrx in &quot;$@&quot;; do # Demonstrate the special property of &quot;for&quot; on a nameref.
typeset -n _arr=$_arrx
(( _result += ${_arr[*]} ))
printf '%s = %d\n' &quot;${!_result}&quot; &quot;$_result&quot; # Demonstrate the special property of ${!ref} on a nameref.
done
}
a=(1 2 3); b=(6 5 4); c=(2 4 6)
sum total a b c
printf 'Final value of &quot;total&quot; is: %d\n' &quot;$total&quot;
&lt;/div&gt;
''typeset -n'' is currently implemented in ksh93, mksh, and Bash 4.3. Bash and mksh's implementations are quite similar, but much different from ksh93's. See [[#portability_considerations | Portability considerations]] for details. ksh93 namerefs are much more powerful than Bash's.
===== Portability considerations =====
* ''declare'' is not specified by POSIX(r)
* ''declare'' is unique to Bash and totally non-portable with the possible exception of Zsh in Bash compatibility mode. Bash marks the synonym ''typeset'' as obsolete, which in Bash behaves identically to ''declare''. All other Korn-like shells use ''typeset'', so it probably isn't going away any time soon. Unfortunately, being a non-standard builtin, ''typeset'' differs significantly between shells. ksh93 also considers ''typeset'' a special builtin, while Bash does not - even in POSIX mode. If you use ''typeset'', you should attempt to only use it in portable ways.
* **todo** nameref portability...
===== See also =====
* [[syntax:arrays]]
* [[commands:builtin:readonly]]
* [[commands:builtin:unset]]
* [[http://austingroupbugs.net/view.php?id=351 | declaration commands]] will change the behavior of certain builtins such as ''export'' in the next version of POSIX.</textarea>
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<ul class="list-inline"><li><span class="iconify text-muted" data-icon="mdi:file-document-outline"></span> <span title="commands/builtin/declare.txt">commands/builtin/declare.txt</span></li><li><span class="iconify text-muted" data-icon="mdi:calendar"></span> Last modified: <span title="2013/04/14 20:11">2013/04/14 20:11</span></li><li class="text-muted">by <bdi>ormaaj</bdi></li></ul> </span>
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