Frequently used commands

Published on: 22nd Apr 2014

Updated on: 20th Oct 2023

Overview

This page contains the frequently used commands in normal day operations.

Operations

To view the processes that is running,
```
top
```
Or
```
htop
```
Where htop has to be installed with the following command,
```
sudo apt install htop
```
To view memory usage
```
free -h
```
Where the "-h" argument means human readable numbers.
To find the location of a program (where 7z is a compression utility program),
```
whereis 7z
```
To search text in a text file, we will first call cat to load the contents into memory and then pipes (|) the contents of the grep command to do the searching. In the following case, it matches for 'some-search-text' that appears in the 'auth.log' file.
```
sudo cat /var/log/auth.log | grep some-search-text
```
If a file has been gzip (file extension is .gz), then, use zcat to load and decompress the contents and the pipes the contents to grep:
```
sudo zcat /var/log/auth.log.1.gz | grep some-search-text
```
Matching text1 and excluding text2, in this case, you need two grep commands. The first one is matching text1 and after that excluding text2.
```
sudo zcat /var/log/auth.log.1.gz | grep text1 | grep -Ev text2
```
Find out current directory,
```
pwd
```
To search file in sub-directory,
```
find . -name myFileName*
```
To cancel a command, press CONTROL+C.

Server management

To view current server name,
```
hostname
```
To change the host name,
```
sudo hostnamectl set-hostname myserver01
```
Before rebooting the server (this requires to do it for one time only),
```
sudo nano /etc/cloud/cloud.cfg
```
Then, change the following line from "false" to "true".
```
preseve_hostname: true
```
Save it and reboot the server.
To reboot the server,
```
sudo reboot
```
Or
```
sudo shutdown -r
```
To shut down the server,
```
sudo shutdown
```
Viewing the server OS version
```
cat /etc/lsb-release
```
To check if the server requires rebooting after apt update
```
cat /var/run/reboot-required
```

18.Sep.2022: How to find out if the server is listening to port 21.

Run this command: netstat -peanut

And you will see something like this:

tester@ubuntu11:/$ netstat -peanut
(Not all processes could be identified, non-owned process info
will not be shown, you would have to be root to see it all.)
Active Internet connections (servers and established)
Proto Recv-Q Send-Q Local Address           Foreign Address         State       User       Inode      PID/Program name
tcp        0      0 0.0.0.0:80              0.0.0.0:*               LISTEN      0          24969      -
tcp        0      0 0.0.0.0:21              0.0.0.0:*               LISTEN      0          28595      -
tcp        0      0 127.0.0.53:53           0.0.0.0:*               LISTEN      101        22735      -
tcp        0      0 0.0.0.0:22              0.0.0.0:*               LISTEN      0          24780      -
tcp        0    240 192.168.101.11:22       192.168.101.1:62289     ESTABLISHED 0          27000      -
tcp6       0      0 :::8080                 :::*                    LISTEN      1000       26678      955/node /home/test
tcp6       0      0 :::80                   :::*                    LISTEN      0          24970      -
tcp6       0      0 :::8084                 :::*                    LISTEN      1000       26679      984/node /home/test
tcp6       0      0 :::22                   :::*                    LISTEN      0          24791      -
udp        0      0 127.0.0.53:53           0.0.0.0:*                           101        22647      -
udp        0      0 10.0.2.15:68            0.0.0.0:*                           100        17443      -

From the result, we confirm that the server is listening on port 21.

20.Oct.2023: view the current time setting in the server
```
timedatectl
```
To view all available time zones.
```
timedatectl list-timezones
```
To change the time zone to UTC+0,
```
timedatectl set-timezone UTC
```

Daemon/service management

To start, stop, restart or check status of a daemon

sudo systemctl [start|stop|restart|status] service_name

For example, check the fail2ban service

sudo systemctl status fail2ban

OR equivalent command:

sudo service fail2ban status

To view all services

sudo systemctl -l

sudo service --status-all

To view failed services
```
sudo systemctl --failed
```
Search for 'nginx' service
```
sudo service --status-all | grep nginx
```

apt command

To view installed packages

sudo apt list --installed
sudo apt list -i

List the installed of any 'php':

sudo apt list --installed php*

View the list of upgradable packages,
```
apt list --upgradeable
```
To upgrade the installed components,
```
sudo apt update
sudo apt full-upgrade
```
- 'full-upgrade' option will install the update and the remove the unused package (to recover storage space).
To see if you need to reboot the server after full upgrade,
```
sudo cat /var/run/reboot-required
```
To cleanup the unused components and recover some spaces,
```
sudo apt --purge autoremove
```
For example, to remove 'php',
```
sudo apt remove php7.3-common -y
sudo apt purge php7.3-common -y
```
- 'remove' is to uninstall the package.
- 'purge' is to delete the installed files.
Finally, upgrade the OS if you have it.
```
sudo do-release-upgrade
```

Personalize your console command by using `alias`.

Edit your console profile,

sudo nano ~/.bashrc

Then, add the following (my frequently used commands) to the bottom of the file,

alias f2b='sudo iptables -S | grep f2b'
alias f2b-log='sudo tail -n 30 /var/log/fail2ban.log'
alias auth-log='sudo tail /var/log/auth.log -n 100'
alias check-reboot='cat /var/run/reboot-required'

After saving the changes, you will be able to use the alias. For example, to view the IP address that blocked by fail2ban.

sudo f2b

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Author

Lau Hon Wan, software developer.