Process Management
Finding processes
ps aux | grep someprocess
top
Press ? or H while running top to get a list of options
Changing process priority
The nice command is used to the prioritize the process to the kernel.
The values range from -19 to +19 where a high value is being very nice => lower priority so:
-19 == very nice == lowest priority
+19 == not nice == highest priority
A child process inherits the priority from the parent.
To change the priority after it was initially modified, we would use the renice command:
# Increase `nice` value by 10
nice -n -10 /bin/slowprocess
# Decrease value by 10
nice -n 10 /bin/slowprocess
# Change the priority of process IDs 987 and 32,
# and all processes owned by users daemon and root,
# to be one greater (+1, one increment "nicer") than its current value.
renice +1 987 -u daemon root -p 32
Useful settings for priority are:
- 20: the affected processes will run only when nothing else in the system needs the resources.
- 0: the default.
- any negative value: will make things go very fast, at the expense of other processes.
Killing Processes
| Signal Name | Number | Description |
|---|---|---|
SIGHUP | 1 | This is known as the Hangup (HUP) signal. It stops the designated process and restarts it with the same PID. |
SIGINT | 2 | This is the Interrupt (INT) signal. It is a weak kill signal that isn’t guaranteed to work, but it works in most cases. |
SIGQUIT | 3 | This is known as the core dump . It terminates the process and saves the process information in memory, and then it saves this information in the current working directory to a file named core . |
SIGTERM | 15 | This is the Termination (TERM) signal. It is the kill command’s default kill signal. |
SIGKILL | 9 | This is the absolute kill signal. It forces the process to stop by sending the process’s resources to a special device, /dev/null . |
To kill a process:
# Kill specific process ID
kill -9 6996
# Kill process by name
killall -9 zombieprocess
Run process in the background
Using & at the end of the command
aprocess 1234 &
Moving a process to the foreground
fg 1234
Scheduling processes
We can use two tools:
at- a daemon process useful for one-time job to be run in the future.
at 7:20am
at> /root/myscanningscript
at now + 20 minutes
at> /root/myscanningscript
at 7:20pm 06/25/2019
crond- more suited for every day/week type of jobs.