Instead of a calculator, I tend to use IPython for those quotidian bits of "mental" arithmetic:

In [1]: 17 * 22.2 Out [1]: 377.4

However, I often forget to actually start IPython, resulting in me running the following in my shell:

$ 17 * 22.2 zsh: command not found: 17

Whilst I could learn do this maths within Zsh itself, I would prefer to dump myself into IPython instead — being able to use "_" and Python modules generally is just too useful.

After following this pattern too many times, I put together the following snippet that will detect whether I have prematurely attempted a calculation inside zsh and pretend that I ran it in IPython all along:

zmodload zsh/pcre math_regex='^[\d\-][\d\.\s\+\*\/\-]*$' function math_precmd() { if [ "${?}" = 0 ] then return fi if [ -z "${math_command}" ] then return fi if whence -- "$math_command" 2>&1 >/dev/null then return fi if [ "${math_command}" -pcre-match "${math_regex}" ] then echo ipython -i -c "_=${math_command}; print _" fi } function math_preexec() { typeset -g math_command="${1}" } typeset -ga precmd_functions typeset -ga preexec_functions precmd_functions+=math_precmd preexec_functions+=math_preexec

For example:

lamby@seriouscat:~% 17 * 22.2 zsh: command not found: 17 377.4 In [1]: _ + 1 Out [1]: 378.4

(Canonical version from my zshrc.d)