Here is the start of an inventory of the extra features that dash and its builtins have over POSIX.1-2017.

• Supports local scope of variables and options via a local builtin (from original Ash in 1989).
• Supports (in some builds) some -V/-E options as aliases for -o vi, -o emacs.
• Supports a -p / -o privileged option.
• A -l option to start as login shell
• -I as alias to -o ignoreeof
• its echo supports the -n option
• its read supports a -p prompt option
• its printf supports %f/e/E/g/G.
• its [/test support -ef, -nt, -ot, <, >
• its ulimit supports many more options that those specified by POSIX
• handling of MAIL, MAILCHECK, MAILPATH variables.

Here is the start of an inventory of the extra features that dash and its builtins have over what's specified by POSIX.1-2017.

• Supports local scope of variables and options via a local builtin (from original Ash in 1989).
• Supports (in some builds) some -V/-E options as aliases for -o vi, -o emacs.
• -I as alias to -o ignoreeof (all options have a single-letter equivalent so it can be tracked in $-, which you can make local with local).
• Supports a -p / -o privileged option.
• A -l option to start as login shell
• its echo supports the -n option
• its read supports a -p prompt option
• its printf supports %f/e/E/g/G directives.
• both its echo and printf support \e to output an ESC.
• its [/test support -ef, -nt, -ot, <, > operators
• its ulimit supports many more options that those specified by POSIX
• handling of MAIL, MAILCHECK, MAILPATH variables.