Useful Linux Commands

A practical reference for system administration, user management, networking, and more — commands we rely on regularly in our work.

1. VPN Management

1.1 Activate a VPN Connection

The following command activates a VPN connection named VPN1, prompting for credentials if required:

sudo nmcli con up id 'VPN1' --ask

1.2 Check VPN Connection Status

Verify whether the VPN1 connection is currently active:

sudo nmcli con show --active | grep 'VPN1'

2. File & Directory Management

2.1 Get the Size of Home Subdirectories

Calculate and display the disk usage of each subdirectory under /home:

sudo du -sh /home/*

Flag Description

du Reports disk usage of files and directories.

-s Displays a summary total for each directory (suppresses subdirectory detail).

-h Formats output in a human-readable form (KB, MB, GB, etc.).

2.2 Sync Only New or Modified Files Between Hosts

Copy files to a remote machine, preserving timestamps and excluding temporary or trajectory files:

rsync -avzu --progress --exclude='*tmp*' --exclude='mdcrd*' /home/user/data/ user@192.168.1.10:/home/user/backup/

Flag Description

-a Archive mode — preserves symbolic links, permissions, timestamps, and other attributes.

-v Verbose output to track which files are being transferred.

-u Skips files that are newer on the destination than the source.

-z Compresses data during transfer to reduce bandwidth usage.

--progress Displays real-time transfer progress for each file.

--exclude Prevents files matching the specified pattern from being copied.

3. User Management

3.1 Add a New User

Creates a new user account with the specified username:

sudo adduser username

3.2 Grant Administrator Privileges

Adds the user to the sudo group, giving them administrative access:

sudo adduser username sudo

3.3 Delete a User and Their Files

Removes a user account along with their home directory and mail spool:

sudo userdel -r username

3.4 List All Users

Displays all user accounts registered on the system:

cat /etc/passwd

3.5 Terminate All Processes of a User

Forcefully kills every active process belonging to the specified user:

sudo killall -9 -u username

4. System Utilities

4.1 Delay a Command by a Specified Time

Use sleep to postpone command execution. The example below runs ls after a one-hour delay:

sleep 1h && ls

You can substitute 1h with values such as 30m (minutes) or 90s (seconds).

4.2 Enable SSH on Ubuntu

Install the OpenSSH server to allow secure remote connections:

sudo apt update

sudo apt install openssh-server

The SSH service typically starts automatically after installation. To confirm it is running:

sudo systemctl status ssh

If the service is inactive, start it manually and enable it to launch at boot:

sudo systemctl start ssh

sudo systemctl enable ssh

4.3 Synchronize the System Clock

Step 1. On the machine with the correct time, retrieve the current timestamp:

date --iso-8601='second'

Step 2. On the machine with the incorrect time, set it using the value obtained above:

sudo date --set 2026-03-02T09:55:21+03:30

5. Troubleshooting & System Updates

5.1 Resolve a GLIBC Version Error

If you encounter the error:

/lib/x86_64-linux-gnu/libm.so.6: version 'GLIBC_2.29' not found

Try installing the required Fortran runtime library:

sudo apt-get install libgfortran3

Note: libgfortran3 is not officially supported on Ubuntu 20.04. If the package is unavailable, copy libgfortran.so.3 from an older Ubuntu system using the commands below.

sudo su

cd /usr/lib/x86_64-linux-gnu

scp -r username@<remote-IP>:/usr/lib/x86_64-linux-gnu/libgfortran.so.3 .

6. Process Control

6.1 Suspend a Running Process

Pause a process using its PID (e.g., 20419). The process is stopped but remains in memory:

sudo kill -STOP 20419

6.2 Resume a Suspended Process

Continue execution of a previously paused process:

sudo kill -CONT 20419

7. Converting Python Scripts to Standalone Binaries

PyInstaller packages a Python script and all its dependencies into a single executable binary, which can be distributed and run on machines without a Python installation.