shell: mention ability to tune up PS* variables (in particular PS1)

[yarikoptic]

apparently some terminals/shells have "short" version of the PS1 which shows only the current directory name (not a full path etc). This makes it difficult sometimes to have a coherent mental picture about actual location within the hierarchy, leading to all kinds of misdemeanor. Knowing where you are is kinda important for reproducibility

[yarikoptic]

If shell is not minimalistic posh, and bash or fixed up dash, I use a trick with PS4 more and more.

Having

export PS4='> $(date +%T.%N) [$$] '
set -x

results in every ran command visualized with timestamp (to see how long it took for a previous one to finish) and a return code (if used without set -e)