Command Line

A concise cdecl reference for understanding and constructing complex C type declarations.

The install command copies files while setting ownership, permissions and target directories in one step. It is commonly used in build scripts and deployment tasks. Copy a file with permissions Create a directory with permissions For system locations such as /usr/local/bin, you may need appropriate privileges. Always check the target path before copying files.

dpkg is the lower-level package tool used by Debian-based Linux systems. It works directly with .deb files and the installed package database. Install a local .deb file If dependencies are missing, fix them with: List installed packages Check whether a package is installed Find which package owns a file For normal package installation from repositories, … Read more

apt-get is a command-line package tool used on Debian-based Linux systems. It is commonly used to install, update, remove and search for software packages. Update package information This refreshes the local package index. It does not upgrade packages by itself. Install a package Remove a package Upgrade installed packages On modern systems, apt is often … Read more

The tac command prints a file from bottom to top. It is similar to cat, but it reverses the order of lines. Basic use This can be useful when reading logs or text files where the newest lines are at the end. Example with a log file For large log files, tools like tail, less, … Read more

umount detaches a mounted filesystem from the Linux directory tree. It is commonly used for USB drives, external disks, network mounts and temporary mount points. Check mounted filesystems Unmount by mount point This unmounts the filesystem currently attached at /mnt/backup. When the device is busy If unmounting fails with a busy message, check which process … Read more

A safer short note on using fsck to check Linux filesystems and why device selection matters.

chkconfig was commonly used on older Linux distributions to view and manage which services start at different runlevels. On many modern systems, systemctl has replaced it. List service settings Modern systemd equivalent Use chkconfig mainly when working with older Red Hat, CentOS or similar systems that still use the older service model.

The printenv command prints environment variables for the current shell session. These variables can affect paths, language settings, application configuration and script behaviour. Show all environment variables Show one variable This is useful when troubleshooting scripts that behave differently for different users or services.

Modern notes on shred, secure file deletion, SSD wear levelling, snapshots, encryption, and device sanitisation.