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4. Examples for different shells

Below we provide an set of examples for some of the more common shells. We start with zsh as it provides several facilities that make our job much easier. We will then progress through increasingly difficult examples.

In all the examples we test the environment variable $TERM to make sure we only apply the escapes to xterms. We test for $TERM=xterm*; the wildcard is because some variants (such as rxvt) can set $TERM=xterm-color.

We should make an extra comment about C shell derivatives, such as tcsh and csh. In C shells, undefined variables are fatal errors. Therefore, before testing the variable $TERM, it is necessary to test for its existence so as not to break non-interactive shells. To achieve this you must wrap the examples below in something like:

  if ($?TERM) then
      ...
  endif

(In our opinion this is just one of many reasons not to use C shells. See Csh Programming Considered Harmful for a useful discussion).

The examples below should be used by inserting them into the appropriate shell initialisation file; i.e. one that is sourced by interactive shells on startup. In most cases this is called something like .shellrc (e.g. .zshrc, .tcshrc, etc).

4.1 zsh

zsh provides some functions and expansions, which we will use:

precmd ()   a function which is executed just before each prompt
chpwd ()    a function which is executed whenever the directory is changed
\e          escape sequence for escape (ESC)
\a          escape sequence for bell (BEL)
%n          expands to $USERNAME
%m          expands to hostname up to first '.'
%~          expands to directory, replacing $HOME with '~'

There are many more expansions available: see the zshmisc man page.

Thus, the following will set the xterm title to "username@hostname: directory":

case $TERM in
    xterm*)
        precmd () {print -Pn "\e]0;%n@%m: %~\a"}
        ;;
esac

This could also be achieved by using chpwd() instead of precmd(). The print builtin works like echo, but gives us access to the % prompt escapes.

4.2 tcsh

tcsh has some functions and expansions similar to those of zsh:

precmd ()   a function which is executed just before each prompt
cwdcmd ()   a function which is executed whenever the directory is changed
%n          expands to username
%m          expands to hostname
%~          expands to directory, replacing $HOME with '~'
%#          expands to '>' for normal users, '#' for root users
%{...%}     includes a string as a literal escape sequence

Unfortunately, there is no equivalent to zsh's print command allowing us to use prompt escapes in the title string, so the best we can do is to use shell variables (in ~/.tcshrc):

switch ($TERM)
    case "xterm*":
        alias precmd 'echo -n "\033]0;${HOST}:$cwd\007"'
        breaksw
endsw

However, this gives the directory's full path instead of using ~. Instead you can insert the string in the prompt:

switch ($TERM)
    case "xterm*":
        set prompt="%{\033]0;%n@%m:%~\007%}tcsh%# "
        breaksw
    default:
        set prompt="tcsh%# "
        breaksw
endsw

which sets a prompt of "tcsh% ", and an xterm title and icon of "username@hostname: directory". Note that the "%{...%}" must be placed around escape sequences (and cannot be the last item in the prompt: see the tcsh man page for details).

4.3 bash

bash supplies a variable $PROMPT_COMMAND which contains a command to execute before the prompt. This example sets the title to username@hostname: directory:

PROMPT_COMMAND='echo -ne "\033]0;${USER}@${HOSTNAME}: ${PWD}\007"'

where \033 is the character code for ESC, and \007 for BEL.

Note that the quoting is important here: variables are expanded in "...", and not expanded in '...'. So $PROMPT_COMMAND is set to an unexpanded value, but the variables inside "..." are expanded when $PROMPT_COMMAND is used.

However, $PWD produces the full directory path. If we want to use the ~ shorthand we need to embed the escape string in the prompt, which allows us to take advantage of the following prompt expansions provided by the shell:

\u          expands to $USERNAME
\h          expands to hostname up to first '.'
\w          expands to directory, replacing $HOME with '~'
\$          expands to '$' for normal users, '#' for root
\[...\]     embeds a sequence of non-printing characters

Thus, the following produces a prompt of bash$ , and an xterm title of username@hostname: directory:

case $TERM in
    xterm*)
        PS1="\[\033]0;\u@\h: \w\007\]bash\\$ "
        ;;
    *)
        PS1="bash\\$ "
        ;;
esac

Note the use of \[...\], which tells bash to ignore the non-printing control characters when calculating the width of the prompt. Otherwise line editing commands get confused while placing the cursor.

4.4 ksh

ksh provides little in the way of functions and expansions, so we have to insert the escape string in the prompt to have it updated dynamically. This example produces a title of username@hostname: directory and a prompt of ksh$ .

case $TERM in
    xterm*)
        HOST=`hostname`
        PS1='^[]0;${USER}@${HOST}: ${PWD}^Gksh$ '
        ;;
    *)
        PS1='ksh$ '
        ;;
esac

However, $PWD produces the full directory path. We can remove the prefix of $HOME/ from the directory using the ${...##...} construct. We can also use ${...%%...} to truncate the hostname:

HOST=`hostname`
HOST=${HOST%%.*}
PS1='^[]0;${USER}@${HOST}: ${PWD##${HOME}/}^Gksh$ '

Note that the ^[ and ^G in the prompt string are single characters for ESC and BEL (can be entered in emacs using C-q ESC and C-q C-g).

4.5 csh

This is very difficult indeed in csh, and we end up doing something like the following:

switch ($TERM)
    case "xterm*":
        set host=`hostname`
        alias cd 'cd \!*; echo -n "^[]0;${user}@${host}: ${cwd}^Gcsh% "'
        breaksw
    default:
        set prompt='csh% '
        breaksw
endsw

where we have had to alias the cd command to do the work of sending the escape sequence. Note that the ^[ and ^G in the string are single characters for ESC and BEL (can be entered in emacs using C-q ESC and C-q C-g).

Notes: on some systems hostname -s may be used to get a short, rather than fully-qualified, hostname. Some users with symlinked directories may find `pwd` (backquotes to run the pwd command) gives a more accurate path than $cwd.

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