Can dash replace bash as my default shell?

Question

From what I read, dash can be made as a shell by using the command

~$ chsh shirish
Password: 
Changing the login shell for shirish
Enter the new value, or press ENTER for the default
    Login Shell [/bin/bash]: 

I just need dash installed and give the path of the login shell /bin/dash. If I do that it doesn't read .bashrc but probably is reading .profile. This is my .bashrc could I just cut and paste .bashrc and dash will behave exactly as bash did in reference to history size, the kind of prompt and the cowsay instance I want or not?

Here's my .bashrc.

$ cat .bashrc
# ~/.bashrc: executed by bash(1) for non-login shells.
# see /usr/share/doc/bash/examples/startup-files (in the package bash-doc)
# for examples

# If not running interactively, don't do anything
case $- in
    *i*) ;;
      *) return;;
esac

# don't put duplicate lines or lines starting with space in the history.
# See bash(1) for more options
HISTCONTROL=ignoreboth

# append to the history file, don't overwrite it
shopt -s histappend

# for setting history length see HISTSIZE and HISTFILESIZE in bash(1)
HISTSIZE=3000
HISTFILESIZE=3000
HISTTIMEFORMAT="%F %T"

# check the window size after each command and, if necessary,
# update the values of LINES and COLUMNS.
shopt -s checkwinsize

# If set, the pattern "**" used in a pathname expansion context will
# match all files and zero or more directories and subdirectories.
#shopt -s globstar

# make less more friendly for non-text input files, see lesspipe(1)
#[ -x /usr/bin/lesspipe ] && eval "$(SHELL=/bin/sh lesspipe)"

# set variable identifying the chroot you work in (used in the prompt below)
if [ -z "${debian_chroot:-}" ] && [ -r /etc/debian_chroot ]; then
    debian_chroot=$(cat /etc/debian_chroot)
fi

# set a fancy prompt (non-color, unless we know we "want" color)
case "$TERM" in
    xterm-color) color_prompt=yes;;
esac

# uncomment for a colored prompt, if the terminal has the capability; turned
# off by default to not distract the user: the focus in a terminal window
# should be on the output of commands, not on the prompt
#force_color_prompt=yes

if [ -n "$force_color_prompt" ]; then
    if [ -x /usr/bin/tput ] && tput setaf 1 >&/dev/null; then
    # We have color support; assume it's compliant with Ecma-48
    # (ISO/IEC-6429). (Lack of such support is extremely rare, and such
    # a case would tend to support setf rather than setaf.)
    color_prompt=yes
    else
    color_prompt=
    fi
fi

# enable bash completion in interactive shells
if ! shopt -oq posix; then
  if [ -f /usr/share/bash-completion/bash_completion ]; then
    . /usr/share/bash-completion/bash_completion
  elif [ -f /etc/bash_completion ]; then
    . /etc/bash_completion
  fi
fi

if [ "$color_prompt" = yes ]; then
    PS1='${debian_chroot:+($debian_chroot)}\[\033[01;32m\]\u@\h\[\033[00m\]:\[\033[01;34m\]\w\[\033[00m\]\$ '
else
    PS1='${debian_chroot:+($debian_chroot)}\t \u@\h:\w\$ '
fi
unset color_prompt force_color_prompt

# If this is an xterm set the title to user@host:dir
case "$TERM" in
xterm*|rxvt*)
    PS1="\[\e]0;${debian_chroot:+($debian_chroot)}\u@\h: \w\a\]$PS1"
    ;;
*)
    ;;
esac

# enable color support of ls and also add handy aliases
if [ -x /usr/bin/dircolors ]; then
    test -r ~/.dircolors && eval "$(dircolors -b ~/.dircolors)" || eval "$(dircolors -b)"
    alias    ls='ls --color=auto'
    alias   cls='clear'
    alias    ll='ls -l'
    alias aptfn='sudo aptitude forget-new'
    alias  aptn="aptitude search '~N'"
    alias    gi='bash /home/shirish/git-info.sh'
    alias  apto='aptitude search ~o'
    alias  grep='grep --color=auto'
    alias  aptc="aptitude search '~c'"
    alias  copy="rsync --progress -ravz"
    alias   vlc="vlc -vv"
    alias   tor="/home/shirish/.local/share/torbrowser/tbb/x86_64/tor-browser_en-US/start-tor-browser"

    #alias dir='dir --color=auto'
    #alias vdir='vdir --color=auto'
    #alias grep='grep --color=auto'
    #alias fgrep='fgrep --color=auto'
    #alias egrep='egrep --color=auto'
fi

# some more ls aliases

#alias la='ls -A'
#alias l='ls -CF'

# Alias definitions.
# You may want to put all your additions into a separate file like
# ~/.bash_aliases, instead of adding them here directly.
# See /usr/share/doc/bash-doc/examples in the bash-doc package.

#if [ -f ~/.bash_aliases ]; then
#    . ~/.bash_aliases
#fi

# from http://askubuntu.com/questions/16428/showing-a-cowsay-fortune-in-every-new-terminal-session

if [ -x /usr/games/cowsay -a -x /usr/games/fortune ]; then
    fortune | cowsay

Solution 1

Shells have two uses: to run scripts, and for an interactive command line.

Dash is designed to be a fast, efficient shell for scripting. It has next to no nice features for interactive use. It doesn't have fancy prompts, command line editing features or command history.

You can make dash your login shell. It will read ~/.profile, same as other Bourne-style shells (bash reads it unless the bash-specific ~/.bash_profile is present). Dash also reads ENV when it's running interactively, like most modern Bourne-style shells.¹

For interactive use, use a more capable shell: fish, zsh or bash. Most of the code in your .bashrc is specific to bash, apart from the alias definitions which would work in other shells.

Recommended background reading: Difference between Login Shell and Non-Login Shell?

¹ _{Some older Bourne-style shells read ENV when they started non-interactively, but I don't think any modern version does it. Ash stopped doing this in 2001.}

Solution 2

Lots of the commands in your .bashrc are bash-specific. Things like HISTCONTROL aren't relevant to dash. Likewise for the command completion. Aliases will work however.

Read the manpage for dash and check the manpage for each thing you're doing in your .bashrc to see whether it's applicable to dash.

The real question you're asking is not very clear: do you just want dash installed as /bin/sh, or do you also really want dash as your login shell? If the latter, then chsh is the right way; otherwise you just need to install dash and the installation will ask whether you want dash to be the default system shell (and then choose "yes"). If you want to change your answer, run dpkg-reconfigure dash.

Solution 3

You can easily try things out for yourself and see if they are working without changing your current login shell.

Apart from /etc/profile and ~/.profile, dash will read the file pointed to by an environment variable ENV when starting interactively so:

ENV=~/.bashrc dash

or better make a copy of your ~/.bashrc to ~/.dashrc and comment out/delete what you don't need/is not working. Once you have everything to you liking you can add a line

ENV=~/.dashrc

to your ~/.profile and change your default shell.

From man dash:

 ...   If the environment variable ENV is set on entry to an interac‐
 tive shell, or is set in the .profile of a login shell, the shell next
 reads commands from the file named in ENV.  Therefore, a user should
 place commands that are to be executed only at login time in the
 .profile file, and commands that are executed for every interactive
 shell inside the ENV file.  To set the ENV variable to some file, place
 the following line in your .profile of your home directory

       ENV=$HOME/.shinit; export ENV

 substituting for “.shinit” any filename you wish.

Comments

[Harold Fischer]

Could you give an example where using the ENV variable is a "bad idea"? I set ENV to my .shrc file on FreeBSD and I don't want do it if it can cause problems.

[Gilles 'SO- stop being evil']

@HaroldFischer See my footnote. Typically: MYVAR=some-different-value sh -c 'do something with a changed $MYVAR' — if the $ENV file sets MYVAR, this won't work since it'll run with the value of MYVAR overridden by $ENV.

[Harold Fischer]

I can't replicate that behavior in dash or ash. The $ENV file is not sourced in a subshell unless I explicitly use sh -i -c 'echo "$MYVAR"'; my $ENV file isn't overriding anything unless I explicitly force the shell to behave interactively

[Gilles 'SO- stop being evil']

@HaroldFischer You're right. Ash stopped reading $ENV when not interactive in 2001. There doesn't seem to be a modern sh version that reads $ENV when not interactive. Thanks, I've corrected my answer.