function require_curl() { which "curl" &>/dev/null; }

function debug() { ((DEBUG)) && echo ">>> $*"; }
...
debug "Starting database import."

URL=${URL:-http://localhost:8080}

• Use hashtags for commands to easily find them again in your history.

<ESC> e

<ESC> .

reset

ls -l =curl

<C-e>d

zmv -Q '(**/)(*).txt' '$1$2.md'

dry run:

zmv -n 'maillog(*)' 'maillog_old${1}'

• http://www.zzapper.co.uk/zshtips.html (local copy, probably outdated: https://wiki.sysop.cafe/resources/zshtips.html)
• http://www.strcat.de/zsh/#tipps