ShellScript to send Mattermost notification is not working - bash

Question

I want to send a Message into a Mattermost channel with the help of a ShellScript/WebHook/cURL. The following code is the function to send the Message.
function matterSend() {
ENDPOINT=https://url.to.Mattermost/WebhookID
USERNAME="${USER}"
PAYLOAD=$(cat <<'EOF'
'payload={
"username" : "${USERNAME}",
"channel" : "TestChannel",
"text" : "#### Test to \n
| TestR | TestS | New Mode |
|:-----------|:-----------:|-----------------------------------------------:|
| ${2} | ${3} | ${1} :white_check_mark: |
"
}'
EOF
)
echo "CURL: curl -i -X POST -d ${PAYLOAD} ${ENDPOINT}"
curl -i -X POST -d "${PAYLOAD}" "${ENDPOINT}"
}
As you can see, when I ECHO the command I get:
curl -i -X POST -d 'payload={
"username" : "TestUser",
"channel" : "TestChannel",
"text" : "#### Test to \n
| TestR | TestS | New Mode |
|:-----------|:-----------:|-----------------------------------------------:|
| ${2} | ${3} | ${1} :white_check_mark: |
"
}' https://url.to.Mattermost/WebhookID
If I paste that code directly into the terminal and execute it, it works. But when I run the script with the help of a Jenkins-Job I get the Error:
Unable to parse incoming data","message":"Unable to parse incoming
data".
Why is it not working?

Answer 1

Without knowledge of the API you are connecting to, I would guess that you need
# Drop function keyword, indent body
matterSend() {
# Lowercase variable names; declare them local
local endpoint=https://url.to.Mattermost/WebhookID
local username=$USER
# Pro tip: don't use a variable for the payload if it's effectively static
payload=$(cat <<-__EOF
payload={
"username" : "$username",
"channel" : "TestChannel",
"text" : "#### Test to \\n| TestR | TestS | New Mode |\\n|:-----------|:-----------:|-----------------------------------------------:|\\n| ${2} | ${3} | ${1} :white_check_mark: |\\n"
}
__EOF
)
echo "CURL: curl -i -X POST -d $payload $endpoint"
curl -i -X POST -d "$payload" "$endpoint"
}
Replacing the newlines inside the "text" element with \n (and doubling the backslash because the here document is now being interpreted by the shell when it's assigned) is mildly speculative; perhaps remove the remaining newlines, too. The real beef is removing the misplaced literal single quotes around the payload.
Maybe also explore printf for formatting fixed-width tabular text.
The here document's <<-__EOF uses the unquoted separator __EOF and the dash before it says to remove any tabs from the beginning of each line. Needless to say, the indentation on those lines consists of a literal tab character.

Answer 2

Generating JSON (or XML, or other structured formats) via string concatenation leads to pain and suffering. Instead, use a tool that actually understands the format.
Using a compliant generator such as jq means you no longer need to be responsible for putting \ns in the data (as multi-character strings), changing "s in text to \", or any of the other otherwise-necessary munging.
matterSend() {
# Lowercase variable names; declare them local
local endpoint=https://url.to.Mattermost/WebhookID
local username=$USER
local text="#### Test to
| TestR | TestS | New Mode |
|:-----------|:-----------:|-----------------------------------------------:|
| ${2} | ${3} | ${1} :white_check_mark: |
"
payload=$(jq --arg username "$username" \
--arg channel "TestChannel" \
--arg text "$text" \
'{"username": $username, "channel": $channel, "text": $text}')
# Advice: use "set -x" if you want to trace the commands your script would run.
# ...or at least printf %q, as below; avoids misleading output from echo.
# printf '%q ' curl -i -X POST -d "$payload" "$endpoint" >&2; echo >&2
curl -i -X POST -d "$payload" "$endpoint"
}