ash.txt

Ash sets at least the following shell variables:

PPID
PWD
OPTIND

Ash sets the following command in certain situations or depending on the build
cconfiguration:

OLDPWD
RANDOM
CDPATH

Ash reads and honors the following environment variables (some support
dependens on build settings):

HOME
HISTFILE
HISTFILESIZE
MAIL
MAILPATH
PS1
PS2
PS4
LC_ALL
LC_CTYPE
LANG

Escape sequences supported in $PS1:

\a   ASCII bell character (07)
\e   ASCII escape character (033)
\n   newline
\r   carriage return
\\   backslash
\nnn char with octal code nnn
\$   if the effective UID is 0, a #, otherwise a $
\w   current working directory, with $HOME abbreviated with a tilde
     Note: we do not support $PROMPT_DIRTRIM=n feature
\W   basename of the current working directory, with $HOME abbreviated with
     a tillde
\h   hostname up to the first '.'
\H   hostname
\u   username
\[   begin a sequence of non-printing characters
\]   end a sequence of non-printing characters
\A   current time in 24-hour HH:MM format
\t   for (weak) compatibility with other shells only: does the same as \A
\T   same as \t
\@   same as \t

Also, command substitution with backquotes is supported in $PS1.

The read builtin supports the following options:

-r              Do not interpret '\' specially
-s              Turn off echo (tty only)
-n NCHARS       Read NCHARS max
-p PROMPT       Display PROMPT on stderr (if input is from tty)
-t SECONDS      Timeout after SECONDS (tty or pipe only)
-u FD           Read from given FD instead of fd 0

Some notes on built-in commands:

"cd" fully supports the "-" argument, $OLDPWD and $CDPATH as explained in the
POSIX standard. Many people do not know about this useful POSIX feature.