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diff --git a/linux-command-line/shell-basics/index.md b/linux-command-line/shell-basics/index.md new file mode 100644 index 0000000..c0cf75b --- /dev/null +++ b/linux-command-line/shell-basics/index.md @@ -0,0 +1,327 @@ +--- +title: Shell Basics +--- + +# Shell Basics + +The shell is your primary interface to a Linux system. Everything — file management, process control, system administration — flows through it. Before graphical tools, before IDEs, there was the shell. Mastering it is non-negotiable. + +**Prerequisites:** None +**Estimated time:** 4-6 hours of practice + +--- + +## What is a Shell? + +A shell is a program that reads commands from your input and executes them. It's both a **command interpreter** (runs programs) and a **programming language** (scripts). + +Common shells: +- **bash** — Bourne Again Shell, the default on most Linux systems +- **zsh** — Extended bash with better completion and prompting +- **sh** — POSIX shell, the lowest common denominator +- **fish** — Friendly interactive shell, not POSIX-compatible + +When you open a terminal, you're running a **terminal emulator** (alacritty, kitty, xterm) which hosts a **shell process**. The terminal draws pixels; the shell interprets commands. + +```bash +# Check your current shell +echo $SHELL + +# List available shells +cat /etc/shells + +# The shell prompt typically shows: +# username@hostname:current_directory$ +``` + +> **Key distinction:** The terminal and the shell are different things. You can run bash inside alacritty, or inside a Linux TTY, or over SSH. The shell doesn't care what's displaying its output. + +--- + +## Navigating the Filesystem + +Linux has a single filesystem tree rooted at `/`. Everything is a file — devices, processes, sockets. + +### Essential Navigation Commands + +```bash +pwd # Print working directory — where you are +ls # List files in current directory +ls -la # Long format, show hidden files (dotfiles) +ls -lh # Human-readable sizes (K, M, G) +cd /etc # Change to /etc +cd ~ # Change to home directory (also just: cd) +cd - # Change to previous directory +cd .. # Go up one level +``` + +### The Filesystem Hierarchy + +| Path | Purpose | +|------|---------| +| `/` | Root of everything | +| `/home` | User home directories | +| `/etc` | System configuration files | +| `/var` | Variable data (logs, databases, mail) | +| `/tmp` | Temporary files (cleared on reboot) | +| `/usr` | User programs and data (read-only) | +| `/usr/bin` | Most user commands | +| `/usr/local` | Locally installed software | +| `/dev` | Device files | +| `/proc` | Process information pseudo-filesystem | +| `/sys` | Kernel/device information pseudo-filesystem | +| `/opt` | Optional/third-party software | + +```bash +# The filesystem is a tree. Explore it: +ls / +ls /etc +ls /dev + +# Everything is a file: +file /dev/null # character special device +file /etc/hostname # ASCII text +file /usr/bin/ls # ELF 64-bit executable +``` + +--- + +## File Operations + +### Creating, Copying, Moving, Deleting + +```bash +# Create +touch newfile.txt # Create empty file (or update timestamp) +mkdir mydir # Create directory +mkdir -p path/to/nested/dir # Create nested directories + +# Copy +cp file.txt backup.txt # Copy file +cp -r mydir/ mydir_backup/ # Copy directory recursively + +# Move / Rename +mv old.txt new.txt # Rename +mv file.txt ~/Documents/ # Move to another directory + +# Delete +rm file.txt # Remove file (no trash, no undo) +rm -r mydir/ # Remove directory recursively +rm -ri mydir/ # Interactive — asks before each deletion +rmdir emptydir/ # Remove only if empty +``` + +> **Warning:** `rm` is permanent. There is no trash can. `rm -rf /` will destroy your entire system if you have the permissions. Always double-check your path before pressing Enter. + +### Viewing Files + +```bash +cat file.txt # Print entire file to stdout +less file.txt # Paginated viewer (q to quit, / to search) +head -n 20 file.txt # First 20 lines +tail -n 20 file.txt # Last 20 lines +tail -f /var/log/syslog # Follow a file in real-time (great for logs) +wc -l file.txt # Count lines +``` + +`less` is your friend. Use it for anything longer than a screenful. Key bindings inside `less`: +- `Space` / `b` — page down / up +- `/pattern` — search forward +- `n` / `N` — next / previous search match +- `g` / `G` — go to top / bottom +- `q` — quit + +--- + +## Permissions + +Every file has an **owner**, a **group**, and a set of **permissions** (read, write, execute) for each of: owner, group, others. + +```bash +ls -l file.txt +# -rw-r--r-- 1 ahmed users 1234 Mar 31 10:00 file.txt +# │├──┤├──┤├──┤ +# │ │ │ └── others: read only +# │ │ └────── group: read only +# │ └─────────── owner: read + write +# └───────────── file type: - = regular, d = directory, l = symlink +``` + +### chmod — Change Permissions + +```bash +# Symbolic notation +chmod u+x script.sh # Add execute for owner +chmod go-w file.txt # Remove write for group and others +chmod a+r file.txt # Add read for all + +# Octal notation (most common) +chmod 755 script.sh # rwxr-xr-x — owner full, others read+execute +chmod 644 file.txt # rw-r--r-- — owner read+write, others read +chmod 600 secret.key # rw------- — owner only +``` + +**Octal cheat sheet:** r=4, w=2, x=1. Add them up per group. +- `7` = rwx (4+2+1) +- `6` = rw- (4+2) +- `5` = r-x (4+1) +- `4` = r-- (4) +- `0` = --- (0) + +### chown — Change Ownership + +```bash +chown ahmed file.txt # Change owner +chown ahmed:users file.txt # Change owner and group +chown -R ahmed:users mydir/ # Recursive +``` + +### Directories and Execute Permission + +The execute bit on directories means something different: it grants **access** (the ability to `cd` into it and access files within). A directory with `r--` lets you list filenames but not read the files. A directory with `--x` lets you access files by name but not list them. + +--- + +## Pipes and Redirection + +This is where the shell gets powerful. The Unix philosophy: small programs that do one thing well, connected together. + +### Redirection + +Every process has three standard streams: +- **stdin** (0) — input +- **stdout** (1) — normal output +- **stderr** (2) — error output + +```bash +# Redirect stdout to a file +ls > filelist.txt # Overwrite +ls >> filelist.txt # Append + +# Redirect stderr +gcc broken.c 2> errors.txt # Errors go to file +gcc broken.c 2>&1 # Stderr merges into stdout + +# Redirect both +command > output.txt 2>&1 # Both to same file +command &> output.txt # Bash shorthand for the above + +# Discard output +command > /dev/null 2>&1 # Silence everything + +# Read stdin from a file +sort < unsorted.txt +``` + +### Pipes + +Pipes connect stdout of one command to stdin of the next. + +```bash +# Find the 10 largest files in /var/log +du -sh /var/log/* 2>/dev/null | sort -rh | head -10 + +# Count unique IP addresses in a log +cat access.log | awk '{print $1}' | sort | uniq -c | sort -rn | head -20 + +# Find running processes matching a pattern +ps aux | grep nginx | grep -v grep + +# Chain as many as you need +cat /etc/passwd | cut -d: -f1 | sort | head -5 +``` + +> **Useless use of cat:** `cat file | grep pattern` can be written as `grep pattern file`. But in pipelines with many stages, starting with `cat` can be clearer. Don't stress about it early on — clarity matters more than micro-optimization. + +### Command Substitution + +Use the output of one command inside another: + +```bash +# Using $() — preferred +echo "Today is $(date +%Y-%m-%d)" +files=$(ls *.txt) + +# Using backticks — older style, harder to nest +echo "Today is `date +%Y-%m-%d`" +``` + +--- + +## Wildcards (Globbing) + +The shell expands patterns before passing them to commands: + +```bash +* # Match any characters (except leading dot) +? # Match exactly one character +[abc] # Match a, b, or c +[0-9] # Match any digit +[!abc] # Match anything except a, b, c +** # Match directories recursively (bash: shopt -s globstar) + +# Examples +ls *.txt # All .txt files +ls file?.log # file1.log, fileA.log, etc. +ls *.{jpg,png} # All .jpg and .png files (brace expansion, not globbing) +rm /tmp/test_* # All files starting with test_ in /tmp +``` + +--- + +## Getting Help + +```bash +man ls # Manual page for ls (press q to quit) +man -k "copy files" # Search manual descriptions +ls --help # Quick help (most GNU tools) +type ls # Is it a builtin, alias, or binary? +which python # Full path of a command +whatis ls # One-line description +apropos network # Search man pages by keyword +``` + +**Reading man pages:** The synopsis section uses conventions: +- `[optional]` — square brackets mean optional +- `REQUIRED` — caps or no brackets means required +- `...` — can be repeated +- `a | b` — choose one + +--- + +## Exercises + +### Drill Exercises + +1. Navigate to `/etc` and list all files starting with `host`. What do they contain? +2. Create a directory structure: `~/practice/project/{src,docs,tests}` in one command +3. Create a file, set its permissions to 750, then explain who can do what with it +4. Use `ls -la /` and identify which entries are regular files, directories, and symlinks +5. Redirect the output of `ls /nonexistent` to `/dev/null` while keeping error messages visible + +### Pipeline Challenges + +6. Count how many lines in `/etc/passwd` contain the string `nologin` +7. List the 5 largest files in `/usr/bin` by size (hint: `ls -lS` or `du`) +8. Extract just the usernames from `/etc/passwd` (field 1, colon-separated) and sort them +9. Find all environment variables that contain the string `PATH` (hint: `env | grep`) +10. Get the total line count of all `.conf` files in `/etc/` (hint: `wc -l`, `cat`) + +### Mini-Project + +**Build a quick reference card:** Create a file `~/cheatsheet.md` that contains the 20 commands you found most useful from this section, organized by category, with a one-line description and example for each. This is your cheatsheet — you'll extend it throughout the course. + +--- + +## Key Takeaways + +- The shell is a command interpreter and a programming environment +- Everything in Linux is a file, organized in a single tree from `/` +- Permissions are the foundation of Linux security — understand `rwx` and octal +- Pipes and redirection are the core of composing commands +- `man` and `--help` are always available — read them first, search the web second +- Practice in a real terminal. Reading about commands is not the same as using them. + +--- + +**Next:** [File Management](../file-management/) — find, locate, tar, rsync, and more |