I know there is ctrl+c, but sometimes that doesn't work. In Ubuntu desktop I can just close the terminal window and open a new one when this happens, but how would this be solved using the CLI in Ubuntu server (without restarting the box)?
CTRL+C will send
SIGINT to the application. The application can configure a handler for this signal or it can ignore the signal. By default there is no handler and
SIGINT will kill the application.
You can use CTRL+\ which will send
SIGQUIT. This will also generate a core dump if the core limit is not zero.
You can suspend the process and return to shell with CTRL+Z, this will stop the execution of the process and return to the shell prompt. The process will be in memory and it will be available as a job in the current shell. You can then use
kill -SIGNAL %% or
kill -SIGNAL %<job_ID> to send a signal to that job. E.g. to kill the last job use
kill -9 %%
If none of them are working you can always send
SIGTERM, then, as last resort,
SIGKILL which will terminate any process. This signal as any other signals must be sent as the same user as the process you are trying to stop or as root. To send
SIGKILL to process, first find the process with
ps aux or
ps -edf, then run
kill -SIGKILL <process_ID>, where the
<process_ID> is the
PID column in
The signals can not be delivered if the process is in an uninterruptible call. Uninterruptible calls are kernel functions that can not be stopped and usually happen because of a bad driver (e.g. a driver that is not reentrant). A process that is in uninterruptible sleep can not be stopped until the call gets completed or the server is rebooted.
If a process becomes a zombie, it will not use any resources only taking space in the process table. A zombie process can not receive signals.
The list of signals for the current architecture can be found with
See the man pages of
bash. To see a man page use something like:
If you have full console access, you can do Alt-F1..12 and get a new console.
From there, you can do a process listing like follows:
ps aux | grep <process-name>
Then do a
kill on the process ID:
kill -9 <pid>
If you don't have full console access, just open another terminal window (perhaps via PuTTY or similar), and do the above process listing and kill steps.
pkill to be easier to use than
kill with an explicit process ID.
Also, instead of starting with signal 9 (SIGKILL), consider starting with the default SIGTERM (15). This will give the process a chance to terminate gracefully (if it can).
pkill -15 thing and
pkill thing should be equivalent.
Here's how that would work. Let's say that ntpd is hung.
What are the processes? (You can skip to pkill if you believe you won't have false positives).
$ pgrep -fl ntp 1034 /usr/sbin/ntpd 1037 /usr/sbin/ntpd
Kill the processes:
$ pkill ntpd
pgrep again to see if it was successful.
If not, eventually move to
Cntrl + \ worked for me (where Cntrl + c or z didn't work)
I usually follow the steps below when Ubuntu 16.04 freezes in my Lenovo Thinkpad E470.
- Go to no-gui mode (Cntrl + Alt + F1).
topcommand to see which process has high CPU usage (above 50% or so). Note the PID of the process.
- Come out of
topcommand (press q).
- [Only if you know
killcommand] Kill the process using
kill -9 <process ID you noted down>.
- Go back to UI mode (Cntrl + Alt + F7)