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Linux Process Management and Service Commands for DevOps Engineers

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This page explains how DevOps engineers monitor, control, and troubleshoot running processes and system services in Linux-based production environments.

Process & Service Architecture

In modern Linux distributions (like Ubuntu 20.04+, CentOS 7+, and RHEL), systemd is the primary init system and service manager. It is responsible for starting services and managing processes from boot to shutdown.

graph TD
    A[Kernel] --> B[systemd (PID 1)]
    B --> C[Service Units]
    B --> D[User Processes]
    C --> E[sshd.service]
    C --> F[httpd.service]
    E --> G[Process ID: 1234]
    F --> H[Process ID: 5678]
    style B fill:#f9f,stroke:#333,stroke-width:2px

systemctl Command

The systemctl command is used to check the status, start, stop, restart, reload, enable, and disable services.

In Linux, we have the sshd service, which is used to connect to Linux servers.

Check Service Status

systemctl status service_name

● sshd.service - OpenSSH server daemon
   Loaded: loaded (/usr/lib/systemd/system/sshd.service; enabled; vendor preset: enabled)
   Active: active (running) since Thu 2023-04-20 12:34:07 GMT; 11h ago
     Docs: man:sshd(8)
           man:sshd_config(5)
 Main PID: 26927 (sshd)
    Tasks: 3
   Memory: 41.3M
   CGroup: /system.slice/sshd.service
           ├─ 2703 sshd: root [priv]
           ├─ 2704 sshd: root [net]
           └─26927 /usr/sbin/sshd -D

Start Service

systemctl start service_name

httpd is the most popular web server.

[opc@new-k8s ~]$ systemctl status httpd
● httpd.service - The Apache HTTP Server
   Loaded: loaded (/usr/lib/systemd/system/httpd.service; disabled; vendor preset: disabled)
   Active: inactive (dead)
     Docs: man:httpd(8)
           man:apachectl(8)

[opc@new-k8s ~]$ sudo systemctl start httpd

[opc@new-k8s ~]$ sudo systemctl status httpd
● httpd.service - The Apache HTTP Server
   Loaded: loaded (/usr/lib/systemd/system/httpd.service; disabled; vendor preset: disabled)
   Active: active (running) since Fri 2023-04-21 00:02:49 GMT; 6s ago
     Docs: man:httpd(8)
           man:apachectl(8)
 Main PID: 22505 (httpd)
   Status: "Processing requests..."
    Tasks: 6
   Memory: 25.6M
   CGroup: /system.slice/httpd.service
           ├─22505 /usr/sbin/httpd -DFOREGROUND
           ├─22506 /usr/sbin/httpd -DFOREGROUND
           ├─22507 /usr/sbin/httpd -DFOREGROUND
           ├─22508 /usr/sbin/httpd -DFOREGROUND
           ├─22509 /usr/sbin/httpd -DFOREGROUND
           └─22510 /usr/sbin/httpd -DFOREGROUND

Apr 21 00:02:49 new-k8s systemd[1]: Starting The Apache HTTP Server...
Apr 21 00:02:49 new-k8s systemd[1]: Started The Apache HTTP Server.

Stop Service

systemctl stop service_name

[opc@new-k8s ~]$ sudo systemctl stop httpd

[opc@new-k8s ~]$ sudo systemctl status httpd
● httpd.service - The Apache HTTP Server
   Loaded: loaded (/usr/lib/systemd/system/httpd.service; disabled; vendor preset: disabled)
   Active: inactive (dead)
     Docs: man:httpd(8)
           man:apachectl(8)

Apr 21 00:02:49 new-k8s systemd[1]: Starting The Apache HTTP Server...
Apr 21 00:02:49 new-k8s systemd[1]: Started The Apache HTTP Server.
Apr 21 00:04:11 new-k8s systemd[1]: Stopping The Apache HTTP Server...
Apr 21 00:04:12 new-k8s systemd[1]: Stopped The Apache HTTP Server.

Restart Service

systemctl restart service_name

[opc@new-k8s ~]$ sudo systemctl status httpd
● httpd.service - The Apache HTTP Server
   Loaded: loaded (/usr/lib/systemd/system/httpd.service; disabled; vendor preset: disabled)
   Active: active (running) since Fri 2023-04-21 00:05:06 GMT; 14s ago
     Docs: man:httpd(8)
           man:apachectl(8)
 Main PID: 23901 (httpd)
   Status: "Total requests: 0; Current requests/sec: 0; Current traffic:   0 B/sec"
    Tasks: 6
   Memory: 25.6M
   CGroup: /system.slice/httpd.service
           ├─23901 /usr/sbin/httpd -DFOREGROUND
           ├─23906 /usr/sbin/httpd -DFOREGROUND
           ├─23907 /usr/sbin/httpd -DFOREGROUND
           ├─23908 /usr/sbin/httpd -DFOREGROUND
           ├─23909 /usr/sbin/httpd -DFOREGROUND
           └─23910 /usr/sbin/httpd -DFOREGROUND

Apr 21 00:05:06 new-k8s systemd[1]: Starting The Apache HTTP Server...
Apr 21 00:05:06 new-k8s systemd[1]: Started The Apache HTTP Server.

[opc@new-k8s ~]$ sudo systemctl restart httpd

[opc@new-k8s ~]$ sudo systemctl status httpd
● httpd.service - The Apache HTTP Server
   Loaded: loaded (/usr/lib/systemd/system/httpd.service; disabled; vendor preset: disabled)
   Active: active (running) since Fri 2023-04-21 00:06:17 GMT; 16s ago
     Docs: man:httpd(8)
           man:apachectl(8)
  Process: 24883 ExecStop=/bin/kill -WINCH ${MAINPID} (code=exited, status=0/SUCCESS)
 Main PID: 24904 (httpd)
   Status: "Total requests: 0; Current requests/sec: 0; Current traffic:   0 B/sec"
    Tasks: 6
   Memory: 25.1M
   CGroup: /system.slice/httpd.service
           ├─24904 /usr/sbin/httpd -DFOREGROUND
           ├─24905 /usr/sbin/httpd -DFOREGROUND
           ├─24906 /usr/sbin/httpd -DFOREGROUND
           ├─24907 /usr/sbin/httpd -DFOREGROUND
           ├─24908 /usr/sbin/httpd -DFOREGROUND
           └─24909 /usr/sbin/httpd -DFOREGROUND

Apr 21 00:06:17 new-k8s systemd[1]: Starting The Apache HTTP Server...
Apr 21 00:06:17 new-k8s systemd[1]: Started The Apache HTTP Server.

Enable Service

If we restart our Linux system, services will be stopped and will not start automatically. To start a service automatically on boot, we need to enable it.

systemctl enable service_name

[opc@new-k8s ~]$ sudo systemctl status httpd
● httpd.service - The Apache HTTP Server
   Loaded: loaded (/usr/lib/systemd/system/httpd.service; disabled; vendor preset: disabled)
   Active: active (running) since Fri 2023-04-21 00:06:59 GMT; 1min 35s ago
     Docs: man:httpd(8)
           man:apachectl(8)
  Process: 25405 ExecStop=/bin/kill -WINCH ${MAINPID} (code=exited, status=0/SUCCESS)
 Main PID: 25413 (httpd)
   Status: "Total requests: 0; Current requests/sec: 0; Current traffic:   0 B/sec"
    Tasks: 6
   Memory: 25.6M
   CGroup: /system.slice/httpd.service
           ├─25413 /usr/sbin/httpd -DFOREGROUND
           ├─25414 /usr/sbin/httpd -DFOREGROUND
           ├─25415 /usr/sbin/httpd -DFOREGROUND
           ├─25416 /usr/sbin/httpd -DFOREGROUND
           ├─25417 /usr/sbin/httpd -DFOREGROUND
           └─25418 /usr/sbin/httpd -DFOREGROUND

Here its mentioned the service is disabled

Loaded: loaded (/usr/lib/systemd/system/httpd.service; disabled; vendor preset: disabled)

[opc@new-k8s ~]$ sudo systemctl enable httpd
Created symlink from /etc/systemd/system/multi-user.target.wants/httpd.service to /usr/lib/systemd/system/httpd.service.

[opc@new-k8s ~]$ sudo systemctl status httpd
● httpd.service - The Apache HTTP Server
   Loaded: loaded (/usr/lib/systemd/system/httpd.service; enabled; vendor preset: disabled)
   Active: active (running) since Fri 2023-04-21 00:06:59 GMT; 4min 54s ago
     Docs: man:httpd(8)
           man:apachectl(8)
 Main PID: 25413 (httpd)
   Status: "Total requests: 0; Current requests/sec: 0; Current traffic:   0 B/sec"
   CGroup: /system.slice/httpd.service
           ├─25413 /usr/sbin/httpd -DFOREGROUND
           ├─25414 /usr/sbin/httpd -DFOREGROUND
           ├─25415 /usr/sbin/httpd -DFOREGROUND
           ├─25416 /usr/sbin/httpd -DFOREGROUND
           ├─25417 /usr/sbin/httpd -DFOREGROUND
           └─25418 /usr/sbin/httpd -DFOREGROUND

Apr 21 00:06:59 new-k8s systemd[1]: Stopped The Apache HTTP Server.
Apr 21 00:06:59 new-k8s systemd[1]: Starting The Apache HTTP Server...
Apr 21 00:06:59 new-k8s systemd[1]: Started The Apache HTTP Server.

Now the service is enabled

Loaded: loaded (/usr/lib/systemd/system/httpd.service; enabled; vendor preset: disabled)

Disable Service

systemctl disable service_name

[opc@new-k8s ~]$ sudo systemctl disable httpd
Removed symlink /etc/systemd/system/multi-user.target.wants/httpd.service.

[opc@new-k8s ~]$ sudo systemctl status httpd
● httpd.service - The Apache HTTP Server
   Loaded: loaded (/usr/lib/systemd/system/httpd.service; disabled; vendor preset: disabled)
   Active: active (running) since Fri 2023-04-21 00:06:59 GMT; 8min ago
     Docs: man:httpd(8)
           man:apachectl(8)
 Main PID: 25413 (httpd)
   Status: "Total requests: 0; Current requests/sec: 0; Current traffic:   0 B/sec"
   CGroup: /system.slice/httpd.service
           ├─25413 /usr/sbin/httpd -DFOREGROUND
           ├─25414 /usr/sbin/httpd -DFOREGROUND
           ├─25415 /usr/sbin/httpd -DFOREGROUND
           ├─25416 /usr/sbin/httpd -DFOREGROUND
           ├─25417 /usr/sbin/httpd -DFOREGROUND
           └─25418 /usr/sbin/httpd -DFOREGROUND

Apr 21 00:06:59 new-k8s systemd[1]: Stopped The Apache HTTP Server.
Apr 21 00:06:59 new-k8s systemd[1]: Starting The Apache HTTP Server...
Apr 21 00:06:59 new-k8s systemd[1]: Started The Apache HTTP Server.

Now the service is disabled

Loaded: loaded (/usr/lib/systemd/system/httpd.service; disabled; vendor preset: disabled)

service Command (Legacy)

service is a "high-level" command used for starting and stopping services in different Unix/Linux systems. Depending on the "lower-level" service manager, service redirects to different binaries.

For example, on CentOS 7 it redirects to systemctl, while on CentOS 6 it directly calls the relative /etc/init.d script. On the other hand, in older Ubuntu releases it redirects to upstart.

service is adequate for basic service management, while directly calling systemctl gives greater control options.

Check Status (service)

service service_name status

[opc@new-k8s ~]$ sudo service httpd status
Redirecting to /bin/systemctl status httpd.service
● httpd.service - The Apache HTTP Server
   Loaded: loaded (/usr/lib/systemd/system/httpd.service; disabled; vendor preset: disabled)
   Active: active (running) since Fri 2023-04-21 00:06:59 GMT; 11min ago
     Docs: man:httpd(8)
           man:apachectl(8)
 Main PID: 25413 (httpd)
   Status: "Total requests: 0; Current requests/sec: 0; Current traffic:   0 B/sec"
   CGroup: /system.slice/httpd.service
           ├─25413 /usr/sbin/httpd -DFOREGROUND
           ├─25414 /usr/sbin/httpd -DFOREGROUND
           ├─25415 /usr/sbin/httpd -DFOREGROUND
           ├─25416 /usr/sbin/httpd -DFOREGROUND
           ├─25417 /usr/sbin/httpd -DFOREGROUND
           └─25418 /usr/sbin/httpd -DFOREGROUND

Apr 21 00:06:59 new-k8s systemd[1]: Stopped The Apache HTTP Server.
Apr 21 00:06:59 new-k8s systemd[1]: Starting The Apache HTTP Server...
Apr 21 00:06:59 new-k8s systemd[1]: Started The Apache HTTP Server.

Stop Service (service)

service service_name stop

[opc@new-k8s ~]$ sudo service httpd stop
Redirecting to /bin/systemctl stop httpd.service

[opc@new-k8s ~]$ sudo service httpd status
Redirecting to /bin/systemctl status httpd.service
● httpd.service - The Apache HTTP Server
   Loaded: loaded (/usr/lib/systemd/system/httpd.service; disabled; vendor preset: disabled)
   Active: inactive (dead)
     Docs: man:httpd(8)
           man:apachectl(8)

Start Service (service)

service service_name start

[opc@new-k8s ~]$ sudo service httpd start
Redirecting to /bin/systemctl start httpd.service

[opc@new-k8s ~]$ sudo service httpd status
Redirecting to /bin/systemctl status httpd.service
● httpd.service - The Apache HTTP Server
   Loaded: loaded (/usr/lib/systemd/system/httpd.service; disabled; vendor preset: disabled)
   Active: active (running) since Fri 2023-04-21 00:20:25 GMT; 3s ago
     Docs: man:httpd(8)
           man:apachectl(8)
 Main PID: 2170 (httpd)
   Status: "Processing requests..."
    Tasks: 6
   Memory: 25.5M
   CGroup: /system.slice/httpd.service
           ├─2170 /usr/sbin/httpd -DFOREGROUND
           ├─2171 /usr/sbin/httpd -DFOREGROUND
           ├─2172 /usr/sbin/httpd -DFOREGROUND
           ├─2173 /usr/sbin/httpd -DFOREGROUND
           ├─2174 /usr/sbin/httpd -DFOREGROUND
           └─2175 /usr/sbin/httpd -DFOREGROUND

Restart Service (service)

service service_name restart

[opc@new-k8s ~]$ sudo service httpd restart
Redirecting to /bin/systemctl restart httpd.service

[opc@new-k8s ~]$ sudo service httpd status
Redirecting to /bin/systemctl status httpd.service
● httpd.service - The Apache HTTP Server
   Loaded: loaded (/usr/lib/systemd/system/httpd.service; disabled; vendor preset: disabled)
   Active: active (running) since Fri 2023-04-21 00:21:58 GMT; 7s ago
     Docs: man:httpd(8)
           man:apachectl(8)
  Process: 3419 ExecStop=/bin/kill -WINCH ${MAINPID} (code=exited, status=0/SUCCESS)
 Main PID: 3433 (httpd)
   Status: "Processing requests..."
    Tasks: 6
   Memory: 25.5M
   CGroup: /system.slice/httpd.service
           ├─3433 /usr/sbin/httpd -DFOREGROUND
           ├─3434 /usr/sbin/httpd -DFOREGROUND
           ├─3435 /usr/sbin/httpd -DFOREGROUND
           ├─3436 /usr/sbin/httpd -DFOREGROUND
           ├─3437 /usr/sbin/httpd -DFOREGROUND
           └─3438 /usr/sbin/httpd -DFOREGROUND

NOTE: We cannot use the service command to enable and disable services.

top Command

The top command provides a dynamic, real-time view of the running system. it displays system summary information and a list of processes currently being managed by the Linux kernel.

top

Interactive Keys inside top:

  • M: Sort processes by Memory usage.
  • P: Sort processes by CPU usage (default).
  • k: Kill a process (will prompt for PID).
  • q: Quit the top interface.
top - 12:34:56 up 11 days,  1:23,  1 user,  load average: 0.00, 0.01, 0.05
Tasks: 123 total,   1 running, 122 sleeping,   0 stopped,   0 zombie
%Cpu(s):  0.3 us,  0.3 sy,  0.0 ni, 99.3 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 st
MiB Mem :   7962.0 total,   5123.4 free,   1234.5 used,   1604.1 buff/cache
MiB Swap:      0.0 total,      0.0 free,      0.0 used.   6450.2 avail Mem 

    PID USER      PR  NI    VIRT    RES    SHR S  %CPU  %MEM     TIME+ COMMAND
  26927 root      20   0  123456  41234   9876 S   0.3   0.5   0:02.45 sshd
   1234 opc       20   0   56789   4567   3456 R   0.1   0.1   0:00.12 top

ps Command

While top is real-time, ps (Process Status) provides a static snapshot of current processes.

View All Processes

DevOps engineers commonly use the -ef flags to see every process on the system in full format.

ps -ef

Filter Specific Processes

Use grep to find a specific service like sshd:

ps -ef | grep sshd

[opc@new-k8s ~]$ ps -ef | grep sshd
root     26927     1  0 Apr20 ?        00:00:02 /usr/sbin/sshd -D
root     32685 26927  0 Apr20 ?        00:00:00 sshd: opc [priv]
opc      32709 32685  0 Apr20 ?        00:00:00 sshd: opc@pts/0

🧠 Quick Quiz — Process & Service Management

#

Which command is used to manage services on modern Linux systems?

📝 Want More Practice?

To strengthen your understanding and prepare for interviews, try the full 20-question practice quiz based on this chapter:

👉 Start Process & Service Management Quiz (20 Questions)

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