mars

ash

ash is the default alpine shell. It is a symlink to the busybox dash.

ls -lh `which ash` `which sh`

lrwxrwxrwx    1 root     root          12 May 17 16:56 /bin/ash -> /bin/busybox
lrwxrwxrwx    1 root     root          12 May 17 16:56 /bin/sh -> /bin/busybox

Pedigree

ash (almquist shell) came around the time of bash. In 90s it was posted to debian as dash, from which the shell in busybox (ash.c) was forked.

Customizing

Two step process. First, create a ~/.profile with

export ENV=$HOME/.ashrc ash

Then put the actual customizations in ~/.ashrc, e.g.

export HISTSIZE=9999999

See shells for the mechanism this is using.

Dynamic xterm title

We can use tty escape sequences to set the title.

printf "\e]0;test\a"
printf "\033]0;test\007" # equivalent

The “\e]0;” escape sequence asks whoever is listening to set title to the part that follows until the next “\a” escape sequence.

Unfortunately I can’t get the printf to work on ash (it works in bash), but that is fine, because while illustrative, we need it to be “dynamic” too.

Fortunately, it works when we put it inside the PS1 (the prompt determinant).

The default prompt for me is “\w $ “ (echo $PS1). So I augment it with the above incantation, so the it runs each time the prompt is displayed.

export PS1="\e]0;$(basename $(pwd))\a\w \$ "

Unfortunately, it still doesn’t update on switching directories. Aha! Of course, that’s because the basename is encoded into the PS1, it does not get reevaluated each time. How?

We can use one of the special sequences that PS1 supports! Let’s use \w

export PS1="\e]0;\w\a\w \$ "

Now is also a good time to look at the documentation (after having solved the issue, as documentation is supposed to be used). Even though not using bash, I can’t think of another place to see the docs except man bash. These following escape sequences look relevant:

\[    begin a sequence of non-printing characters, which could
      be used to embed a terminal control sequence into the prompt
\]    end a sequence of non-printing characters

It seems to work without it, but let’s be nice.

export PS1="\[\e]0;\w\a\]\w \$ "

There is also \t, for the time.

After tweaking to taste, put it in the .ashrc.

Moving on

The busybox ash does not support brace expansion. It is eminently usable, but for the main workstation it is apropos to get a nicer shell. Having already lived in bash and zsh for few years each, it’s time to move on to fish, give it a try.

Update: I did give it a try. It was too friendly. zsh it is then.

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