Bash script can't be run in Shell

Question

I've been attempting to implement an alert script for Zabbix. Zabbix attempts to run the script in Shell for some reason, whilst the script is written in Bash.

#!/bin/bash

# Slack incoming web-hook URL and user name
url='https://hooks.slack.com/services/this/is/my/webhook/'             # example: https://hooks.slack.com/services/QW3R7Y/D34DC0D3/BCADFGabcDEF123
username='Zabbix Notification System'

## Values received by this script:
# To = $1 (Slack channel or user to send the message to, specified in the Zabbix web interface; "@username" or "#channel")
# Subject = $2 (usually either PROBLEM or RECOVERY/OK)
# Message = $3 (whatever message the Zabbix action sends, preferably something like "Zabbix server is unreachable for 5 minutes - Zabbix server (127.0.0.1)")

# Get the Slack channel or user ($1) and Zabbix subject ($2 - hopefully either PROBLEM or RECOVERY/OK)
to="$1"
subject="$2"

# Change message emoji depending on the subject - smile (RECOVERY/OK), frowning (PROBLEM), or ghost (for everything else)
recoversub='^RECOVER(Y|ED)?$'
if [[ "$subject" =~ ${recoversub} ]]; then
        emoji=':smile:'
elif [ "$subject" == 'OK' ]; then
        emoji=':smile:'
elif [ "$subject" == 'PROBLEM' ]; then
        emoji=':frowning:'
else
        emoji=':ghost:'
fi

# The message that we want to send to Slack is the "subject" value ($2 / $subject - that we got earlier)
#  followed by the message that Zabbix actually sent us ($3)
message="${subject}: $3"

# Build our JSON payload and send it as a POST request to the Slack incoming web-hook URL
payload="payload={\"channel\": \"${to//\"/\\\"}\", \"username\": \"${username//\"/\\\"}\", \"text\": \"${message//\"/\\\"}\", \"icon_emoji\": \"${emoji}\"}"
curl -m 5 --data-urlencode "${payload}" $url -A "https://hooks.slack.com/services/this/is/my/web/hook"
~

When I run the script locally using 'bash slack.sh' it sends an empty notification, which I recieve in Slack. When I run the script locally using 'sh slack.sh' I get the following error.

slack.sh: 19: slack.sh: [[: not found
slack.sh: 21: [: unexpected operator
slack.sh: 23: [: unexpected operator
slack.sh: 34: slack.sh: Bad substitution

Thanks for the assistance.

Answer 1

You can force your script to run with bash by adding

#! /bin/bash

if [ -z "$BASH" ]
then
    exec /bin/bash "$0" "$@"
fi
...