Is there a command line interface for slack?

I use this one. Try https://github.com/candrholdings/slack-cli

To install, type

npm install -g slack-cli

To send a message,

slackcli -t <Slack token> -g <Slack group name> -m "Hello, World!"

To send a file,

slackcli -t <Slack token> -g <Slack group name> -f <File name>

To send from standard input,

cat anyFile.txt | slackcli -t <Slack token> -g <Slack group name> -c

You can even set the token as environment variable.

SET SLACK_TOKEN=<Slack token>
slackcli -g yourGroupName -m "Hello, World!"

There is no full-fledged command line interface that I know of, but there is an API, so in principle anybody could start writing one. Slackcat is a first step in that direction.

A potential alternative approach might be installing a text-mode IRC or XMPP client on your servers, and using the Slack/IRC/XMPP gateway.

EDIT/UPDATE: This answer is clearly outdated now – see the comments and other answers.

Here's Slacker, a CLI for Slack using PHP. You'll need your own Slack api token and adjust the configs: https://github.com/TidalLabs/Slacker