Associative arrays in zsh

This is an excerpt from my book “Moving to zsh” which is available for order on the Apple Books Store.

One of the advantages of zsh over bash 3 is the support of “associative arrays,” a data structure known as hash tables or dictionaries in other languages.

In associative arrays, you can store a piece of data, or value with an identifying ‘key’. For example, the associative array userinfo has multiple values, each identified with a key:

% echo $userinfo[name]
armin
% echo $userinfo[shell]
bash
% echo $userinfo[website]
scriptingosx.com

Note: bash 4 also added associative arrays, but they are implemented slightly differently.

Creating associative arrays

In zsh, before you can use a variable as an associative array, you have to declare it as one with

declare -A userinfo

This will tell the shell that the userinfo variable is an associative array. You can also use typeset -A as an alternative syntax. You can verify the type of the variable:

% echo ${(t)userinfo}
association

You can then set the key-value pairs of the userinfo associative array individually:

userinfo[name]="armin"
userinfo[shell]=bash
userinfo[website]="scriptingosx.com"

When you set the value for an existing key again, it will overwrite the existing value:

% echo $userinfo[shell]
bash
% userinfo[shell]=zsh
% echo $userinfo[shell]
zsh

Setting the values for each key is useful in some situations, but can be tedious. You can also set the entire associative array at once. There are two syntaxes for this in zsh:

userinfo=( name armin shell zsh website scriptingosx.com )

This format follows the format ( key1 value1 key2 value2 ...). The other syntax is more verbose and expressive:

userinfo=( [name]=armin [shell]=zsh [website]="scriptingosx.com" )

When you set the associative array variable this way, you are overwriting the entire array. For example, if you set the userinfo for ‘armin’ like above and then set it later like this, the website key and value pair will have been overwritten as well:

% userinfo=( [name]=beth [shell]=zsh )
% if [[ -z $userinfo[website] ]]; then echo no value; fi
no value

If you want to partially overwrite an existing associative array, while leaving the other key/value pairs intact, you can use the += operator:

% userinfo+=( [shell]=fish [website]=fishshell.com )
% echo $userinfo[name]                                           
beth
% echo $userinfo[shell]
fish
% echo $userinfo[website]
fishshell.com

To clear an associative array, you can use:

% userinfo=( )

Retrieving data from an associative array

We have already seen you can get the value for a given key with the ‘subscript’ notation:

% echo $userinfo[name]                                           
beth

When you access the $userinfo variable directly, you will get a normal array of the value:

% echo $userinfo
beth fish fishshell.com

You can also get an array of the keys with this syntax:

% echo ${(k)userinfo}
name shell website

or a list of both keys and values:

% echo ${(kv)userinfo} 
website fishshell.com shell fish name beth

You can use this to copy the data from one associative array to another:

% declare -A otherinfo
% otherinfo=( ${(kv)userinfo )
% echo $otherinfo[name]
beth

You can also use this to loop through all the keys and values of an associated array:

for key value in ${(kv)userinfo}; do
    echo "$key -> $value"
done

#output
website -> fishshell.com
shell -> fish
name -> beth

Limitations

Associative arrays have their uses, but are not as powerful as dictionaries in more powerful languages. In zsh, you cannot nest associative arrays in normal arrays, which limits their use for complex data structures.

There is also no functionality to transfer certain file formats, like XML or property lists directly in to associative arrays or back.

Shell scripting was never designed for complex data structures. When you encounter these limitations, you should move “up” to a higher level language, such as Python or Swift.