I want to quit 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 -x 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 its 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
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.
For example, in your case,
pkill -x Slack will do the trick.
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