Category: Linux CLI

Commands any Linux Sysadmin should learn first.

These are the commands any Linux Sysadmin should learn first.

These are some of the most useful commands for system administration and devops on Linux.

They are command line tolls because most of the actions on system administration you would want to be automated and scheduled

uptime

Time passed since last restart and statistics about system load and users logged in since then.

[dragos@localhost ~]$ uptime
 18:00:51 up 3 days, 20:59,  1 user,  load average: 0.01, 0.04, 0.00

For a nicer look use the pretty option.

uptime -p
[dragos@localhost ~]$ uptime -p
up 3 weeks, 3 days, 21 hours

top

Shows system statistics and resource allocation.

CPU and RAM allocation overview list.

top

alias 

alias command shortcut name = “the actual command”

alias sayhello="echo ""Hello World!"""

[dragos@localhost rex.red]$ alias sayhello="echo ""Hello World!"""
[dragos@localhost rex.red]$ sayhello
Hello World!
[dragos@localhost rex.red]$
This will last while you are still logged in. If you log out and logon again, the alias will be gone.

Permanent alias

Making the alias permanent for your user. You save it in your .bashrc file.

vim ~/.bashrc

Add the alias at the end of the file.

# User specific aliases and functions
#My Aliases
alias sayhello="echo ""Hello World!"""

Save the file – in vim Esc then :wq enter

Make sure you use the file

source ~/.bashrc

Your alias is there even after a reboot.

whereis

Powerful command to locate files or directories on your drive.

Most useful to find binaries of installed programs.

whereis httpd

free

free -h

RAM allocation and usage statistics.

[dragos@localhost ~]$ free
              total        used        free      shared  buff/cache   available
Mem:        1859812      613712      453892       53532      792208     1036984
Swap:       2158588      234220     1924368
[dragos@localhost ~]$ free -h
              total        used        free      shared  buff/cache   available
Mem:          1.8Gi       599Mi       443Mi        52Mi       773Mi       1.0Gi
Swap:         2.1Gi       228Mi       1.8Gi
[dragos@localhost ~]$

service

service service-name action

[dragos@localhost ~]$ service -h
Usage: service < option > | --status-all | [ service_name [ command | --full-restart ] ]

It’s useful to find problems with a daemon or service.

It is also used to (start, stop, restart, try-restart, reload, force-reload, status) various services.

service httpd status

[dragos@localhost ~]$ 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)
  Drop-In: /usr/lib/systemd/system/httpd.service.d
           └─php-fpm.conf
   Active: inactive (dead)
     Docs: man:httpd.service(8)
[dragos@localhost ~]$

[dragos@localhost ~]$ sudo service httpd start
[sudo] password for dragos:
Redirecting to /bin/systemctl start httpd.service
[dragos@localhost ~]$ sudo service httpd enable
The service command supports only basic LSB actions (start, stop, restart, try-restart, reload, force-reload, status). For other actions, please try to use systemctl.
[dragos@localhost ~]$

You can start a service but to enable it on startup use systemctl.

systemctl

sudo systemctl restart httpd
sudo systemctl reload httpd
sudo systemctl stop httpd
sudo systemctl start httpd
sudo systemctl enable httpd
sudo systemctl status httpd

[dragos@localhost ~]$ sudo service httpd start
[sudo] password for dragos:
Redirecting to /bin/systemctl start httpd.service
[dragos@localhost ~]$ sudo service httpd enable
The service command supports only basic LSB actions (start, stop, restart, try-restart, reload, force-reload, status). For other actions, please try to use systemctl.
[dragos@localhost ~]$ sudo systemctl restart httpd
[dragos@localhost ~]$ sudo systemctl reload httpd
[dragos@localhost ~]$ sudo systemctl stop httpd
[dragos@localhost ~]$ sudo systemctl start httpd
[dragos@localhost ~]$ sudo systemctl enable httpd
Created symlink /etc/systemd/system/multi-user.target.wants/httpd.service → /usr/lib/systemd/system/httpd.service.
[dragos@localhost ~]$ sudo systemctl status httpd
● httpd.service - The Apache HTTP Server
   Loaded: loaded (/usr/lib/systemd/system/httpd.service; enabled; vendor preset: disabled)
  Drop-In: /usr/lib/systemd/system/httpd.service.d
           └─php-fpm.conf
   Active: active (running) since Tue 2021-08-31 18:36:43 EEST; 11s ago
     Docs: man:httpd.service(8)
 Main PID: 3778525 (httpd)
   Status: "Running, listening on: port 80"
    Tasks: 213 (limit: 11255)
   Memory: 26.1M
   CGroup: /system.slice/httpd.service
           ├─3778525 /usr/sbin/httpd -DFOREGROUND
           ├─3778526 /usr/sbin/httpd -DFOREGROUND
           ├─3778527 /usr/sbin/httpd -DFOREGROUND
           ├─3778528 /usr/sbin/httpd -DFOREGROUND
           └─3778529 /usr/sbin/httpd -DFOREGROUND

Aug 31 18:36:43 localhost.localdomain systemd[1]: Starting The Apache HTTP Server...
Aug 31 18:36:43 localhost.localdomain systemd[1]: Started The Apache HTTP Server.
Aug 31 18:36:43 localhost.localdomain httpd[3778525]: Server configured, listening on: port 80
lines 1-21/21 (END)

dig

Complete DNS query to find any record related to a domain, name server, root nameserver

dig rex.red

[dragos@localhost ~]$ dig rex.red

; <<>> DiG 9.11.26-RedHat-9.11.26-4.el8_4 <<>> rex.red
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 29461
;; flags: qr rd ra; QUERY: 1, ANSWER: 2, AUTHORITY: 0, ADDITIONAL: 1

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 4000
;; QUESTION SECTION:
;rex.red.                       IN      A

;; ANSWER SECTION:
rex.red.                299     IN      A       172.67.218.197
rex.red.                299     IN      A       104.21.38.42

;; Query time: 347 msec
;; SERVER: 10.4.60.20#53(10.4.60.20)
;; WHEN: Tue Aug 31 10:51:06 EEST 2021
;; MSG SIZE  rcvd: 68

The any parameter will give you all the DNS records of that domain. 

It’s useful to get an overview of the setup of a domain.

[dragos@localhost ~]$ dig rex.red any

; <<>> DiG 9.11.26-RedHat-9.11.26-4.el8_4 <<>> rex.red any
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 28600
;; flags: qr rd ra; QUERY: 1, ANSWER: 5, AUTHORITY: 0, ADDITIONAL: 13

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 4000
;; QUESTION SECTION:
;rex.red.                       IN      ANY

;; ANSWER SECTION:
rex.red.                243     IN      A       104.21.38.42
rex.red.                243     IN      A       172.67.218.197
rex.red.                3597    IN      NS      chan.ns.cloudflare.com.
rex.red.                3597    IN      NS      alec.ns.cloudflare.com.
rex.red.                3786    IN      HINFO   "RFC8482" ""

;; ADDITIONAL SECTION:
chan.ns.cloudflare.com. 11396   IN      A       172.64.32.82
chan.ns.cloudflare.com. 11396   IN      A       173.245.58.82
chan.ns.cloudflare.com. 11396   IN      A       108.162.192.82
chan.ns.cloudflare.com. 1707    IN      AAAA    2803:f800:50::6ca2:c052
chan.ns.cloudflare.com. 1707    IN      AAAA    2a06:98c1:50::ac40:2052
chan.ns.cloudflare.com. 1707    IN      AAAA    2606:4700:50::adf5:3a52
alec.ns.cloudflare.com. 7750    IN      A       172.64.33.59
alec.ns.cloudflare.com. 7750    IN      A       173.245.59.59
alec.ns.cloudflare.com. 7750    IN      A       108.162.193.59
alec.ns.cloudflare.com. 80123   IN      AAAA    2a06:98c1:50::ac40:213b
alec.ns.cloudflare.com. 80123   IN      AAAA    2606:4700:58::adf5:3b3b
alec.ns.cloudflare.com. 80123   IN      AAAA    2803:f800:50::6ca2:c13b

;; Query time: 3322 msec
;; SERVER: 10.4.60.20#53(10.4.60.20)
;; WHEN: Tue Aug 31 10:52:02 EEST 2021
;; MSG SIZE  rcvd: 408

[dragos@localhost ~]$

+nocomments option makes the response more compact by removing the comments of each section and giving you only the actual response.

[dragos@localhost ~]$ dig rex.red any +nocomments

; <<>> DiG 9.11.26-RedHat-9.11.26-4.el8_4 <<>> rex.red any +nocomments
;; global options: +cmd
;rex.red.                       IN      ANY
rex.red.                10      IN      A       172.67.218.197
rex.red.                10      IN      A       104.21.38.42
rex.red.                3364    IN      NS      chan.ns.cloudflare.com.
rex.red.                3364    IN      NS      alec.ns.cloudflare.com.
rex.red.                3553    IN      HINFO   "RFC8482" ""
rex.red.                157     IN      TXT     "google-site-verification=C0S4gecD15hMlSjLUF0bGRNURsYoleGS-v_1zCJ6mr4"
chan.ns.cloudflare.com. 11163   IN      A       173.245.58.82
chan.ns.cloudflare.com. 11163   IN      A       108.162.192.82
chan.ns.cloudflare.com. 11163   IN      A       172.64.32.82
chan.ns.cloudflare.com. 1474    IN      AAAA    2a06:98c1:50::ac40:2052
chan.ns.cloudflare.com. 1474    IN      AAAA    2606:4700:50::adf5:3a52
chan.ns.cloudflare.com. 1474    IN      AAAA    2803:f800:50::6ca2:c052
alec.ns.cloudflare.com. 7517    IN      A       173.245.59.59
alec.ns.cloudflare.com. 7517    IN      A       108.162.193.59
alec.ns.cloudflare.com. 7517    IN      A       172.64.33.59
alec.ns.cloudflare.com. 79890   IN      AAAA    2606:4700:58::adf5:3b3b
alec.ns.cloudflare.com. 79890   IN      AAAA    2803:f800:50::6ca2:c13b
alec.ns.cloudflare.com. 79890   IN      AAAA    2a06:98c1:50::ac40:213b
;; Query time: 26 msec
;; SERVER: 10.4.60.20#53(10.4.60.20)
;; WHEN: Tue Aug 31 10:55:55 EEST 2021
;; MSG SIZE  rcvd: 489

[dragos@localhost ~]$

Looking ony at the TXT DNS records of the domain.

[dragos@localhost ~]$ dig rex.red txt +nocomments

; <<>> DiG 9.11.26-RedHat-9.11.26-4.el8_4 <<>> rex.red txt +nocomments
;; global options: +cmd
;rex.red.                       IN      TXT
rex.red.                26      IN      TXT     "google-site-verification=C0S4gecD15hMlSjLUF0bGRNURsYoleGS-v_1zCJ6mr4"
;; Query time: 0 msec
;; SERVER: 10.4.60.20#53(10.4.60.20)
;; WHEN: Tue Aug 31 10:58:06 EEST 2021
;; MSG SIZE  rcvd: 117

[dragos@localhost ~]$

The default is a which returns the IPv4 of the domain. 

q-type   is one of (a,any,mx,ns,soa,hinfo,axfr,txt,…) [default:a]

ifconfig

List all interfaces

ifconfig -a

Enable the network card interface

ifconfig eth0 down

Disable the network card

ifconfig eth0 up

Configure the network interface

ifconfig eth0 192.168.1.12

ifconfig eth0 netmask 255.255.255.

ifconfig eth0 broadcast 192.168.1.255

netstat

Shows active network connections 

netstat

Show all sockets 

netstat -a

Show statistics for all interfaces

netstat -s

Display interfaces

netstat -i

[dragos@localhost rex.red]$ netstat -h
usage: netstat [-vWeenNcCF] [<Af>] -r         netstat {-V|--version|-h|--help}
       netstat [-vWnNcaeol] [<Socket> ...]
       netstat { [-vWeenNac] -I[<Iface>] | [-veenNac] -i | [-cnNe] -M | -s [-6tuw] } [delay]

        -r, --route              display routing table
        -I, --interfaces=<Iface> display interface table for <Iface>
        -i, --interfaces         display interface table
        -g, --groups             display multicast group memberships
        -s, --statistics         display networking statistics (like SNMP)
        -M, --masquerade         display masqueraded connections

        -v, --verbose            be verbose
        -W, --wide               don't truncate IP addresses
        -n, --numeric            don't resolve names
        --numeric-hosts          don't resolve host names
        --numeric-ports          don't resolve port names
        --numeric-users          don't resolve user names
        -N, --symbolic           resolve hardware names
        -e, --extend             display other/more information
        -p, --programs           display PID/Program name for sockets
        -o, --timers             display timers
        -c, --continuous         continuous listing

        -l, --listening          display listening server sockets
        -a, --all                display all sockets (default: connected)
        -F, --fib                display Forwarding Information Base (default)
        -C, --cache              display routing cache instead of FIB
        -Z, --context            display SELinux security context for sockets

vim

Command line text editors. Especially useful for editing configuration files in emergency situations.

vi file

vim  file

nano file