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I want to quit a certain applications on my Mac using Terminal. For example, how do I kill “Slack”? Do I need its PID number?

No, you do not need to know its PID.

You can use:

pkill Slack

Or:

killall Slack

Note: Be sure to read the manual page for whichever command you choose to use, in order to see the various options available to the command, as may be relevant to is particular usage. In Terminal type e.g. man pkill and press enter, or just type the command and right-click on it, then select: Open man Page

You can install htop then click on the Slack process line and type k9

I'm not sure for Slack, but some Applications will run multiple Processes and you may want to kill just one (I find I often need to do this with iTunes, for example). In that case, you can run

ps -e | grep -i slack

To find all Processes with case-insensitive "slack" in the name. The output should look like (without the header):

PID TTY TIME CMD
649 pts/1 00:00:00 bash

That first column will be your PID. You can then use that to kill specifically that process:

kill -9 649

Replacing 649 with your PID you found from calling ps.

Since I don't yet have the reputation to comment, I'm saying this as a separate answer. pkill without any flags does not match a specific process! For example, running pkill foo would target processes named foo, but would also target processes named foobar. This is because it uses regular expressions.

If you wish to kill a specific process, you can pass it the -x flag. For example, pkill -x foo. This will use exact names instead of regular expressions.

You can use AppleScript to tell the application to quit:

osascript -e 'quit app "Slack"'

this will tell the application to quit and will start all the save and cleanup tasks. Or you can send the TERM signal with pkill but it could be that the application will not shut down cleanly

pkill Slack