Out of neat one-liners I never use but sound good on paper?

    timestamp() {
        while IFS= read -r line; do printf '[%s] %s\n' "$(date '+%F %T.%4N')" "$line"; done
    }

This prepends a timestamp in front of the output of a command, useful for long-running commands, and logging (e.g. "while sleep .1; do echo openssl rand -hex 8; done | timestamp").

Out of scripts I actually use a lot? Boring stuff like "refactor":

    sedescape_regex() {
        printf '%s' "$@" | sed 's/[^^]/[&]/g; s/\^/\\^/g'
    }
    sedescape_subst() {
        printf '%s' "$@" | sed 's/[&/\]/\\&/g'
    }
    
    regex="$(sedescape_regex "$1")"; shift
    subst="$(sedescape_subst "$1")"; shift
    
    grep -rwl \
        --include='*.sh' \
        --include='*.c' --include='*.h' \
        --include='*.asm' --include='*.inc' \
        --include='*.s' --include='*.i' \
        --include='*.ld' \
        --include='*.md' \
        "$regex" "$@" | xargs -t sed -i "s/\<$regex\>/$subst/g"

This one is obviously less useful for languages with namespaces, hence why --include currently only really has C and a few Assembly flavors.

I wish there was a CLI "refactor" tool that supported LSP, without needing a heavy editor to do that.