Linux, terminal and Git cheat sheet (Linux)

1. 1Your machineGetting a Unix shell, a package manager, Git and an editor working, and the words for the parts you are using.6 concepts

   1. 1Linux distributionA packaged Linux system: the kernel plus a package manager, default tools and a release schedule, such as Ubuntu, Debian, Fedora or Arch.
   2. 2Terminal, shell and promptThe terminal is the window.
   3. 3System package managerThe tool that installs, updates and removes software from trusted repositories: apt on Ubuntu and Debian, dnf on Fedora, pacman on Arch and Homebrew on macOS.
   4. 4man, --help and tldrman cmd opens the full manual (search with /, quit with q), cmd --help prints a summary, and tldr cmd shows short practical examples.
   5. 5Git configurationSettings stored in ~/.gitconfig for all repositories, or .git/config for one.
   6. 6VS Code and the code commandVisual Studio Code is a free editor.

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2. 2Shell basicsMoving around, working with files, and how the shell expands patterns, connects commands and finds programs.11 concepts

   1. 1Absolute and relative pathsAn absolute path starts at the root /; a relative path starts from the current folder.
   2. 2File commandsmkdir -p creates folders (and parents), touch creates an empty file, cp -r copies, mv moves or renames, rm deletes and rm -r deletes folders.
   3. 3Globbing and brace expansionThe shell expands * (any characters), ? (one character) and [abc] (one of a set) into matching file names before the command runs.
   4. 4QuotingSingle quotes keep text exactly as written.
   5. 5PipesA pipe | sends the standard output of one command into the standard input of the next.
   6. 6Redirection and the three streamsPrograms read standard input and write standard output and standard error.
   7. 7Environment variablesNamed values inherited by every program the shell starts, such as HOME, USER, EDITOR and PATH.
   8. 8PATHA colon-separated list of folders the shell searches, in order, for commands.
   9. 9Shell startup files and aliasesFiles the shell runs when it starts: ~/.bashrc for interactive Bash, ~/.zshrc for zsh, and login files such as ~/.profile or ~/.zprofile.
   10. 10nano and VimTerminal editors. nano shows its shortcuts: Ctrl+O saves, Ctrl+X exits.
   11. 11Opening files and the clipboardCommands that hand a file to your desktop or move text to and from the clipboard.

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3. 3Text toolsViewing, searching, filtering and transforming text, from logs and CSV files to JSON from an API.8 concepts

   1. 1cat, less, head and tailcat prints a file, less lets you scroll and search it, head and tail show the start and end, and tail -f follows a growing file.
   2. 2grepPrints lines that match a pattern, in files or in piped input.
   3. 3findWalks a folder tree and matches files by name, type, size or modification time, optionally running an action on each.
   4. 4sort, uniq, cut, tr and wcSmall tools that reorder, count, slice and clean lines of text.
   5. 5Regular expressionsA pattern language: . any character, * zero or more, + one or more, ? optional, ^ and $ start and end of line, [0-9] a set, ( ) a group.
   6. 6sedEdits text as it streams through, most often with substitution s/old/new/g.
   7. 7awkProcesses text line by line, splitting each line into fields $1, $2 and so on, with -F to choose the separator.
   8. 8jqA command-line tool for reading, filtering and reshaping JSON.

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4. 4Permissions, processes and packagesWho can read, write and run what, what is running and how to stop it, and how to install tools and language versions cleanly.10 concepts

   1. 1Permission bitsEach file has read, write and execute permissions for its owner, its group and everyone else, shown by ls -l as, for example, rwxr-xr--.
   2. 2chmodChanges permissions, either symbolically (u+x, go-w) or numerically, where read is 4, write 2 and execute 1 for each of owner, group and others.
   3. 3Ownership and chownEvery file belongs to a user and a group.
   4. 4root, sudo, users and groupsroot can do anything.
   5. 5ProcessesA running program, with a process ID, an owner and a parent.
   6. 6Signals and killMessages sent to a process.
   7. 7Jobs: foreground and backgroundcmd & starts a job in the background, Ctrl+Z pauses the foreground job, bg resumes it in the background, fg brings it back and jobs lists them.
   8. 8Installing developer toolsSystem tools come from the system package manager; project libraries come from language managers such as pip and npm, installed per project.
   9. 9Version managersTools that install several versions of a language in your home folder and switch between them by changing PATH, such as nvm for Node and pyenv for Python.
   10. 10Disk and system checksdf -h shows free space per file system, du -sh shows how big folders are, free -h shows memory on Linux and uname -a shows the kernel.

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5. 5Git fundamentalsHow Git stores work, the daily commands, branching and merging, conflicts, and undoing mistakes safely.12 concepts

   1. 1Commits and snapshotsA commit is a snapshot of the whole project plus a message, author and a pointer to its parent commit, identified by a hash.
   2. 2Working tree, staging area and repositoryChanges move from the working tree (your files) to the staging area with git add, then into history with git commit.
   3. 3git log and git diffgit log shows history, git diff shows changes, git show shows one commit and git blame shows who last changed each line.
   4. 4Commit messagesA short imperative summary of about 50 characters, a blank line, then a body explaining why when needed.
   5. 5.gitignoreA file listing patterns Git should not track, such as dependencies, build output, logs and secrets.
   6. 6BranchesA branch is a movable name pointing at a commit.
   7. 7git stashSaves uncommitted changes on a stack and cleans the working tree, so you can switch tasks and restore them later.
   8. 8MergeCombines another branch into the current one, with a merge commit when histories diverged or a fast-forward when they did not.
   9. 9RebaseReplays your commits on top of another branch, creating new commits with new hashes.
   10. 10Merge conflictsWhen two branches change the same lines, Git stops and marks the file with <<<<<<<, ======= and >>>>>>> sections for you to resolve.
   11. 11Undoing changesrestore discards or unstages changes, commit --amend fixes the last local commit, revert adds a commit that undoes another, and reset moves a branch…
   12. 12git reflogA local log of every position HEAD has been at, including commits no branch points to any more.

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6. 6GitHub and collaborationRemotes, SSH keys, the pull request workflow, reviews, forks and the GitHub CLI.9 concepts

   1. 1RemotesNamed links to other copies of the repository.
   2. 2fetch, pull and pushfetch downloads new commits without changing your files, pull fetches and then merges or rebases, and push uploads your commits.
   3. 3SSH keysA key pair: a private key that stays on your machine and a public key (.pub) you give to GitHub or a server.
   4. 4GitHub flowA lightweight workflow: main is always deployable; each change gets a branch, commits, a pull request, review and checks, then a merge and branch deletion.
   5. 5Pull requestA request to merge a branch, with a description, discussion, review and automated checks.
   6. 6Code reviewReading a teammate's change to check correctness, tests, clarity and fit, and leaving comments, suggestions or approval.
   7. 7Forks and syncingA fork is your own copy of someone else's repository on GitHub.
   8. 8GitHub CLIThe official command-line tool for GitHub: repositories, pull requests, issues, releases and Actions runs from the terminal.
   9. 9Secrets in GitPasswords, tokens and keys committed to a repository stay in its history even after you delete the file.

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7. 7Shell scripting and automationSmall, safe Bash scripts, checked by ShellCheck, run on a schedule with cron, and your own dotfiles.10 concepts

   1. 1Shebang and executable scriptsThe first line #!/usr/bin/env bash chooses the interpreter.
   2. 2Exit codesEvery command returns a number: 0 means success, anything else failure.
   3. 3set -euo pipefail-e stops on a failing command, -u treats unset variables as errors, and pipefail makes a pipeline fail if any part fails.
   4. 4Variables and arguments$1, $2 are positional arguments, "$@" is all of them, $# is the count, ${var:-default} gives a fallback and $(cmd) captures output.
   5. 5Conditionals and testsif [[ ... ]]; then ... fi runs code based on a test: -f file exists, -d folder exists, -z empty string, == string match, -gt number comparison.
   6. 6Loopsfor loops over a list or glob; while read -r line reads input line by line.
   7. 7Functions and script structureNamed blocks of commands, called like programs, with local variables.
   8. 8ShellCheckA static analyser for shell scripts that warns about quoting bugs, ignored errors, wrong test syntax and more, each with a numbered explanation page.
   9. 9cronRuns commands on a schedule defined by five fields: minute, hour, day of month, month, day of week.
   10. 10Dotfiles repositoryYour configuration files kept in a Git repository, with an install script that links them into your home with ln -s.

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8. 8SSH, servers and debuggingReaching and securing a Linux server, keeping work alive, reading logs, and checking networks step by step.10 concepts

   1. 1SSH loginssh user@host opens a secure shell on another machine.
   2. 2SSH config fileA file of named Host blocks with the address, user, port and key for each machine.
   3. 3scp and rsyncscp copies files over SSH; rsync synchronises folders, sending only changes.
   4. 4First steps on a new serverUpdate packages, create a sudo user with key login, disable root and password login in /etc/ssh/sshd_config, and enable a firewall allowing only what you…
   5. 5tmuxKeeps terminal sessions running on a machine after you disconnect, with windows and split panes.
   6. 6systemctl and journalctlsystemd starts and supervises services.
   7. 7curlSends HTTP and other requests from the terminal and prints the response.
   8. 8DNS and reachability checksdig or nslookup show what a name resolves to; ping checks whether a host answers; traceroute shows the path.
   9. 9Ports and listening processesA service listens on a port; ss -tlnp (Linux) or lsof -i :PORT shows which process owns it.
   10. 10Environment debugging checklistA fixed order of checks: read the full error, ask what changed, then check which binary runs, PATH, permissions, environment variables, disk, processes, ports…

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9. 9Glossary, A to ZEvery concept in the sheet and the smaller terms it uses, in one list.168 terms

Shell, Git, SSH and your dev setup.