Shell script to send Slack messages over its incoming webhooks

slackmsg

#!/usr/bin/env sh

usage () {
        cat 1>&2 <<ENDUSAGE
Usage: slackmsg [OPTIONS] -t<URL> [--] MESSAGE

Options:
-t|--target URL         The Slack URL to send a message to. (required)
-d|--dry-run            Print the payload and curl request only.
-i|--stdin              Read the MESSAGE from standard input.
-a|--attach JSON        JSON object to attach to the message.
-A|--attachs JSON       JSON array of attachment objects to attach to the message.
-u|--user USER          The username to send the message with.
-c|--channel [@#]CHAN   The channel to send a slack message to.
                        Automatically prefixed with # unless @ or # is included.
ENDUSAGE
}

if test $# -eq 0
then
        usage
        exit 2
fi

dryrun=0
stdin=0
while test $# -gt 0
do
        case "$1" in
        -h|--help) usage; exit 0;;

        -c|--channel) shift; chan="${1}";;
        -c*) chan="${1#-c}";;
        --channel=*) chan="${1#--channel=}";;

        -u|--user) shift; user="${1}";;
        -u*) user="${1#-u}";;
        --user=*) user="${1#--user=}";;

        -t|--target) shift; target="${1}";;
        -t*) target="${1#-t}";;
        --target=*) target="${1#--target=}";;

        -d|--dry-run) dryrun=1;;

        -a|--attach) # singular
                shift
                attachments="$(jq --argjson pre "${attachments:-[]}" --argjson next "[$1]" -n '[$pre, $next]|add')"
                ;;
        -A|--attachs) # plural
                shift
                attachments="$(jq --argjson pre "${attachments:-[]}" --argjson next "$1" -n '[$pre, $next]|add')"
                ;;

        -i|--stdin) stdin=1;;

        --) shift; msg="$*";;
        *) msg="$*"; break;;
        esac

        shift
done

is_dryrun () { test "$dryrun" -eq 1; }

if test -z "$target"
then
        if is_dryrun
        then
                target='https://hooks.slack.com/services/slack/integration'
        else
                echo 'slackmsg: no target URL given' 1>&2
                usage
                exit 2
        fi
fi

case "$chan" in
''|@*|'#'*) ;;
*) chan="#${chan}";;
esac

if test "$stdin" -eq 1
then
        if test -n "$msg"
        then
                echo 'slackmsg: message in args is ignored with --stdin' 1>&2
        fi

        msg="$(cat)"
fi

if test -z "$msg"
then
        echo 'slackmsg: no message' 1>&2
        usage
        exit 2
fi

payload () {
        jq --arg text "$msg" \
                --arg chan "$chan" \
                --arg user "$user" \
                --argjson attachments "${attachments:-null}" \
                -n "$@" '
        { mrkdwn: true, text: $text } |
        if $user != "" then .username = $user else . end |
        if $chan != "" then .channel = $chan else . end |
        if $attachments != null then .attachments = $attachments else . end
        '
}

if test "$dryrun" -eq 1
then
        echo curl --silent --fail -X POST "$target" --data-binary @- '<<EOF' 1>&2
        payload 1>&2
        echo EOF 1>&2
        exit 0
fi

payload -c | curl --silent --fail -X POST "$target" --data-binary @-