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Joins
JOIN types INNER JOIN: returns rows when there is a match in both tables. LEFT JOIN: returns all rows from the left table, even if there are no matches in the right table. RIGHT JOIN: returns all rows from the right table, even if there are no matches in ...
Constraints
What are constraints? Constraints are used to define rules to allow or restrict what values can be stored in columns. MySQL Constraints are: NOT NULL UNIQUE PRIMARY KEY FOREIGN KEY CHECK DEFAULT What is a FOREIGN KEY constraint? The FOREIGN KEY c...
Indexes
Index Types The primary key is always automatically indexed and unique. Indexes by default are saved in a binary tree form (B-TREE). Indexes slow down inserts and updates, so you want to use them carefully on columns that are FREQUENTLY updated. This is ...
Wildcards
What are Wildcards? MySQL provides two wildcard characters for constructing patterns: percentage % and the underscore _. The percentage ( % ) wildcard matches any string of zero or more characters. The underscore ( _ ) wildcard matches any single characte...
File Permissions
How do File Permissions work? Each file and directory has three user based permission groups. owner – The Owner permissions apply only the owner of the file or directory, they will not impact the actions of other users. group – The Group permissions apply...
Archiving and Compression
What are Archives? Archiving is the process of collecting and storing a group of files and directories into one file. The tar utility performs this action and the files are with tar extension. Commands: tar -cf destination_file.tar file_1.* file_2.*... - ...
Device Files
What are Device Files? In Linux various special files can be found under the /dev directory. These files are called device files and behave unlike ordinary files. Two standard types of device files exist. Character devices are devices where that communic...
DD Utility
What is DD command used for? Dd is a very powerful and useful utility available on Unix and Unix-like operating systems. As stated in its manual, its purpose is to convert and copy files. On Unix and Unix-like operating systems like Linux, almost everything i...
Disk Partitioning
What is Disk Partitioning? Disk partitioning is splitting a drive into logical drives or volumes. Each volume on a partitioned disk has its own drive letter and folder structure and also can be formatted with different file systems without affecting other par...
File Systems
What are File Systems? File system is the way storage space on a drive is split. Without it we wouldn't have any way to know where file one file ends and other one starts. An inode is a data structure that describes a file and each file has one. Each file sys...
SSH keys
How to use SSH Keys? To authenticate using SSH keys, a user must have an SSH key pair on their local computer. On the remote server, the public key must be copied to a file within the user’s home directory at ~/.ssh/authorized_keys. This file contains a list ...
Common Linux Commands
General reset - Reset the terminal to default state uptime - Show up-time of the system pwd - Print working directory file file - Outputs the type of the file locate file - Find the the location of the file. File paths are stored in a database and they ar...
How does Git work under the hood?
What is SHA-1? SHA-1 (Secure Hash Algorithm 1) is a cryptographic hash function which takes an input and produces a 160-bit (20-byte) hash value known as a message digest, typically rendered as a hexadecimal number, 40 digits long. SHA-1 is most often used t...
Contributing to Open Source projects
How can I contribute to an Open Source project? A fork is simply a copy of the repo that lives on your own personal Github profile. You can make any changes you want to this fork and experiment all you want with its code, and that’s fine! Any changes you make...
Common Git Commands
Actions git fetch - Downloads new data from a remote repository, but it doesn't integrate any of this new data into your working files. Fetch will never manipulate, destroy, or screw up anything. Example: When you run git fetch, you connect your Git to Sall...
Merge vs Rebase
There are two options for integrating your feature into the main branch: merging directly. rebasing and then merging. How does Git Merge work? Git merge will combine multiple sequences of commits into one unified history. Example: ...
Reset vs Revert
Git Checkout With git checkout, the main ref is still pointing to d. The HEAD ref has been moved, and now points at commit b. The repo is now in a 'detached HEAD' state. git checkout d To move back to the latest commit, just checkout to the branch git ch...
Workflow
What is the Git Workflow? Working directory / Working tree / Workspace holds files in your computer's ordinary format, rather than in the special Git-only format that goes into each new commit you make. The index / staging-area / cache holds the propos...
Shell Scripting
To create a bash script, create a plain file without any extension and put #!/bin/bash at the beginning of the file. Operations $( ) - Command Substitution. Encapsulate the result of command(s) and pass it as an argument to another command. echo $(seq 1 5) ...
Common Docker Commands
Container docker container run Create and start a container from an image --detach (-d) Run the container in the background --name Specify a name for the container --publish (-p) host_port:container_port Make a port available to services outside of Doc...