Interactive Shells and their prompts

2012-03-04

What can we say about what the “-i” option to shell does? It varies according to the shell. Below, I start with PS1 set to “demo$ “, which is not my default. (you can probably work it out from the transcript, but) it might help to know that my default shell is bash (for now). There’s nothing special about ‘pwd’ in the examples below, it’s just a command with a short name that outputs something.

demo$ echo pwd | ksh
/home/drj/hackdexy/content
demo$ echo pwd | bash
/home/drj/hackdexy/content
demo$ echo pwd | ksh -i
$ /home/drj/hackdexy/content
$ 
demo$ echo pwd | bash -i
drj$ pwd
/home/drj/hackdexy/content
drj$ exit
demo$ echo pwd | bash --norc -i
bash-4.2$ pwd
/home/drj/hackdexy/content
bash-4.2$ exit
demo$ echo pwd | sh -i
$ /home/drj/hackdexy/content
$ 
sh: Cannot set tty process group (No such process)
demo$ . ../bin/activate
(hackdexy)demo$ echo pwd | ksh -i
(hackdexy)demo$ /home/drj/hackdexy/content
(hackdexy)demo$ 
(hackdexy)demo$ deactivate

What have we learnt? (when stdin is not a terminal) shells do not issue prompts unless the “-i” option is used. That’s basically what “-i” does. bash, but not ksh, will echo the command. That has the effect of making bash’s output seem more like an interactive terminal session.

Both ‘bash’ and ‘ksh’ changed the prompt. ‘bash’ changed the prompt because it sources my ‘.bashrc’ and that sets PS1; ‘ksh’ changed my prompt, apparently because PS1 is not an exported shell variable, and so ‘ksh’ does not inherit PS1 from its parent process, and so sets it to the default of “$ “. If we stop ‘bash’ from sourcing ‘.bashrc’ by using the ‘–norc’ option (‘–posix’ will do as well) then it too will use its default prompt: the narcissistic “bash-4.2$ “.

‘sh’ (dash on my Ubuntu laptop, apparently), is like ‘ksh’ in that it does not echo the commands. But it does output a narked warning message.

The last command, after using ‘activate’ from virtualenv, demonstrates that ‘activate’ must export PS1. I think that is a bug, but I would welcome comment on this matter.

I guess most people use ‘bash’ and mostly set PS1 their ‘.bashrc’, so wouldn’t notice any of this subtlety (for example, they will not notice that ‘activate’ exports PS1 because the ‘.bashrc’ will set it to something different).

I note that the example login and ENV scripts given in The KornShell Command and Programming Language (p241 1989 edition) set PS1 is the login script only, and do not export it. Meaning that subshells will have the default prompt. I quite like that (it also means that subshells from an ‘activate’d shell will have the ‘activate’ prompt). Why doesn’t anyone do it like that any more?

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