bash: continuously updated iostat (like 'watch iostat') (inspired by https://jrs-s.net/2019/06/04/continuously-updated-iostat/)

watch-iostat.sh

#!/bin/bash
shopt -s extglob

NEWLINE=$'\n'

ANSI_ERASE_ENTIRE_SCREEN=$'\e[2J'
ANSI_RESET_CURSOR_POSITION=$'\e[H'
ANSI_CLEAR="${ANSI_ERASE_ENTIRE_SCREEN}${ANSI_RESET_CURSOR_POSITION}"

die() {
    tput rmcup
    echo "die: ${1}" >&2
    exit 1
}

#open a sub-terminal
tput smcup || die "couldn't enable secondary terminal buffer!"
trap 'tput rmcup' EXIT

#use default args if none are set
if [ $# -eq 0 ]; then
    args=( -p device /dev/disk/by-path/!(*-part[0-9]*) "-hsx" )
else
    args=( "$@" )
fi

S_COLORS=always iostat "${args[@]}" 1 | {
    echo "${ANSI_CLEAR}waiting for data..."

    #we use $REPLY rather than reading directly into a variable because it preserves whitespace

    #read header line
    read || die "couldn't read header!"
    [ ! -z "$REPLY" ] || die "header, got '${REPLY}'"
    header="$REPLY"
    read || die "couldn't read trailing newline after header!"
    [ -z "$REPLY" ] || die "expected a trailing newline after header, got '${REPLY}'"


    #read a whole report and print it
    while true; do
        outbuf=()

        #read an input block
        while true; do
            #read the first line - if it's empty, then we reached the end of the last block
            read || die "couldn't read block start"
            [ ! -z "$REPLY" ] || break

            #keep printing the previous line and reading the next until we hit a newline, indicating
            #the end of the current block
            while [ ! -z "$REPLY" ]; do
                outbuf+=( "$REPLY" )

                read || die "couldn't read next line in block"
            done

            #print final line of block (this is an empty string)
            outbuf+=( "$REPLY" )
        done

        #clear screen and print new buffer contents
        printf '%s' "${ANSI_CLEAR}${header}${NEWLINE}" "${outbuf[@]/#/$NEWLINE}"
    done
}