• IEEE Std 1003.1-2017
• POSIX Programmer's Manual
• Bash Builtins

type命令用于查看某个命令的类型。

type COMMAND_NAME

type which
type command

if command -v which > /dev/null && [ "$(type which)" = "which is a shell builtin" ]; then
    echo "this is zsh $ZSH_VERSION";
elif command -v caller > /dev/null && [ "$(type caller)" = "caller is a shell builtin" ]; then
    echo "this is bash $BASH_VERSION";
elif command -v enum > /dev/null && [ "$(type enum)" = "enum is a special shell builtin" ]; then
    echo "this is ksh $KSH_VERSION";
else
    echo "this is ash";
fi

依据：