Process Management
The ps command is used to view the running processes on a system
By default it only shows the processes that are running in the terminal
# View all processes that belongs to a user
ps -u <username>
# View all the processes running on the system
ps -auxFlags
a: All process on the system (All users)
u: Show user that owns the process
x: Exclude processes in the current terminal
Terminate a Process
To terminate a process the kill command is used
This command takes the process id/ job id as a parameter
# Find process id
pgrep <process-name>
grep -u <username> | grep <process-name>
# Kill process
kill <process-id>If there are multiple processes of a type that we want to kill the pkill command can be used
# Kills all the ping proccesses on the system
pkill -9 <process-name>Foreground & Background Process
Ctrl + C : Sends Interrupt Signal (SIGINT Signal)
Ctrl + Z : Sends Suspend Signal (SIGSTOP Signal)
The job command displays all the running jobs along with their job number
The fg and bg command is used to move process to foreground and background
jobs
bg %<job-no>
fg %<job-no>When a process is in background even though there might be output on the terminal we can still run other commands
The & sign is used with the command to process in the background directly
subl file_name.py &
nohup sleep 1000 &The nohup command prevents the hang-up signal from terminating the process
Signal & Traps
When we use the kill command we are sending the kill signal to the process
There are other signals that can be sent to the process as well
# View all available signals
kill -l | Number | SIG | Meaning |
|---|---|---|
| 1 | SIGHUP | Clean tidy-up |
| 2 | SIGINT | Interrupt |
| 3 | SIGQUIT | Quit |
| 6 | SIGABRT | Abort |
| 9 | SIGKILL | Die Now (cannot be trapped) |
| 14 | SIGALRM | Alarm Clock |
| 15 | SIGTERM | Terminate |
| 18 | SIGCONT | Continue |
| 19 | SIGSTOP | Suspend/ Stop |
When we don’t specify the signal in the kill command SIGTERM is sent to the process It is also called as Soft Kill. In certain cases the process can refuse this signal
When we use Ctrl + Z the SIGSTOP signal is sent to the process
If we use Ctrl + Z the second time SIGCONT signal is send to process
# Forcefully kill the process
kill -KILL %<job-no>
kill -9 %<job-no>In your code we can write logic (Trap) that can be used to detect various signals and catch them. SIGKILL and SIGSTOP Signals cannot be caught, blocked or stopped
In code when a process is interrupted due to error SIGQUIT signal is sent to it