Notes

Service & Process Management

2 min read

Two types of service: Internal - services required by system for startup; external - services installed by the user. Such service run in background without interaction called daemons and are identified as sshd and systemd.

The daemon itself is an init process i.e. has a PID of 1. Monitors and takes care of the other services. The processes can be viewed in /proc/ with numbers known as child processes. Can use systemctl and update-rc.d.

SystemCTL

Upon installing a service like openssh, can use systemctl to start, stop, restart a service or enable to make it start on boot aka startup. Can use ps to see the current processes.

workstation@htb[/htb]$ systemctl start ssh
workstation@htb[/htb]$ systemctl status ssh
 
● ssh.service - OpenBSD Secure Shell server
   Loaded: loaded (/lib/systemd/system/ssh.service; enabled; vendor preset: enabled)
   Active: active (running) since Thu 2020-05-14 15:08:23 CEST; 24h ago
   Main PID: 846 (sshd)
   Tasks: 1 (limit: 4681)
   CGroup: /system.slice/ssh.service
           └─846 /usr/sbin/sshd -D
 
workstation@htb[/htb]$ systemctl enable ssh
workstation@htb[/htb]$ ps -aux | grep ssh
root       846  0.0  0.1  72300  5660 ?        Ss   Mai14   0:00 /usr/sbin/sshd -D
 
workstation@htb[/htb]$ systemctl list-units --type=service
UNIT                                                      LOAD   ACTIVE SUB        
accounts-daemon.service                                    loaded active running
acpid.service                                              loaded active running
apache2.service                                            loaded active running

Kill a Process

You can use the kill, pkill, pgrep or killall commands to kill a certain process. Used:

  • SIGHUP - sent to a process when the terminal that controls it is closed
  • SIGINT - sent to a user presses control + c.
  • SIGQUIT - sent when user presses control + d
  • SIGERM - Program termination
  • SIGKILL - Kill instantly without clean-up operations
  • SIGSTOP - Stop the program; can’t be handled.
  • SIGSTP sent when the user presses control + z.

Running Processes

As mentioned above the cntrl+z command stops the program and puts it into the SIGSTP, which you can access back by using the bg aka background command. To list an background jobs you can use the jobs command.

Likewise, an jobs that are running in the foreground are also displayed in the jobs command, but if you specifically want to see the foreground command then can use fg command.

Command Execution:

  • Semicolon (;): Executes commands sequentially, ignoring errors.
  • Double ampersand (&&): Executes commands sequentially, stopping if an error occurs.
  • Pipe (|): Takes output from one command as input for the next.

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