Declare (Bash)

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The Bash command declare is similar to dim and set in some other language: To declare and maybe initialize a variable.

It can be ommitted for general variables, but is needed in specific cases, like for declaring a name reference, an indexed array or an associative array

$ help declare

declare: declare [-aAfFgilnrtux] [-p] [name[=value] ...]
    Set variable values and attributes.
    Declare variables and give them attributes.  If no NAMEs are given,
    display the attributes and values of all variables.
      -f	restrict action or display to function names and definitions
      -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
      -p	display the attributes and value of each NAME
    Options which set attributes:
      -a	to make NAMEs indexed arrays (if supported)
      -A	to make NAMEs associative arrays (if supported)
      -i	to make NAMEs have the `integer' attribute
      -l	to convert the value of each NAME to lower case on assignment
      -n	make NAME a reference to the variable named by its value
      -r	to make NAMEs readonly
      -t	to make NAMEs have the `trace' attribute
      -u	to convert the value of each NAME to upper case on assignment
      -x	to make NAMEs export
    Using `+' instead of `-' turns off the given attribute.
    Variables with the integer attribute have arithmetic evaluation (see
    the `let' command) performed when the variable is assigned a value.
    When used in a function, `declare' makes NAMEs local, as with the `local'
    command.  The `-g' option suppresses this behavior.
    Exit Status:
    Returns success unless an invalid option is supplied or a variable
    assignment error occurs.

Local scope

When declare is used within a function, the variable gets a local scope, limited to inside this function. To overrule this, use the flag -g to make them globa. E.g.:

   unset j
   declare -gA j

   echo "Length - Inside function: ${#j[@]}"

unset j
echo "Length - Outside function: ${#j[@]}"


Create an associative array:

declare -A j

Declare a reference for another variable:

declare -n j

Declare a variable that can only contain integers + test it:

declare -i i
declare -i j

i="hello, world!"

echo "i: $i"
echo "j: $j"


i: 0
j: 12

When putting the declare and initialization in one line, you actually get an error for the first assignment:

declare -i i="Hello, world!"
declare -i j=12

The output actually puzzles me, but that's for another time and place to figure out:

bash: declare: Hello, world!: syntax error in expression (error token is "!")

You can declare multiple variables at once:

unset site site_bal site_bal_cb site_bal_non_cb site_non_bal
declare -Ag site site_bal site_bal_cb site_bal_non_cb site_non_bal

See also