autoload

Autoload a function by name

$ autoload [function_name]

$ autoload -U [function_name]

$ autoload -Uz [function_name]

$ autoload -Uz $fpath[1]/*(.:t)

$ autoload -Uz compinit && compinit

$ autoload -Uz promptinit && promptinit

autoload [-UXmtz] [name ...]

autoload marks shell function names for deferred loading. Instead of reading a function definition into memory at startup, the shell records only the function name. When the function is first called, the shell searches the directories listed in $fpath for a file matching the function name, reads its definition, and executes it.
This mechanism significantly reduces shell startup time when many functions are available but rarely used. The function file should contain the function body directly (not wrapped in a `function name { }` block for zsh-style autoloading).
autoload is essential for zsh's completion system (compinit), prompt themes (promptinit), and many other standard functions distributed with zsh.

-U

Suppress alias expansion when the function is loaded

Use zsh-style function definitions (default in zsh)

Use ksh-style function definitions

Immediately load and execute the function (used inside the function itself)

Enable execution tracing for the autoloaded function

Treat arguments as patterns for matching function names

Force the function to be loaded immediately without executing it

The function file must be in a directory listed in $fpath. The file name must exactly match the function name. Functions autoloaded with -U will not have aliases expanded in their definitions, which is strongly recommended to avoid unexpected behavior. Only available as a shell builtin in zsh and ksh.

autoload originates from ksh (Korn Shell), designed by David Korn at Bell Labs in the 1980s. Zsh adopted and extended the concept, making it central to its modular function system. The -U and -z flags are zsh-specific additions.

zsh(1), source(1), compinit(1)