Command Reference

Comprehensive reference for commands covered in this course.

File System Commands

`pwd` - Print Working Directory

Syntax: pwd [options]

Common Options:

-P - Physical path (resolve symlinks)

Example:

$ pwd
/home/student

`ls` - List Directory Contents

Syntax: ls [options] [directory]

Common Options:

-l - Long format (permissions, owner, size, date)
-a - Include hidden files (starting with .)
-h - Human-readable sizes
-R - Recursive listing
-t - Sort by modification time

Examples:

$ ls -lh
total 16K
drwxr-xr-x 2 student student 4.0K Feb  5 12:00 documents
-rw-r--r-- 1 student student  123 Feb  5 12:00 notes.txt

$ ls -la
total 32K
drwxr-xr-x 5 student student 4.0K Feb  5 12:00 .
drwxr-xr-x 3 root     root     4.0K Feb  4 00:00 ..
-rw-r--r-- 1 student student  123 Feb  5 12:00 notes.txt
drwx------ 2 student student 4.0K Feb  5 12:00 .ssh

`cd` - Change Directory

Syntax: cd [directory]

Special Paths:

~ - Home directory
- - Previous directory
.. - Parent directory
. - Current directory

Examples:

$ cd /var/log
$ pwd
/var/log

$ cd ~
$ pwd
/home/student

$ cd -
/var/log

`mkdir` - Make Directory

Syntax: mkdir [options] <directory>

Common Options:

-p - Create parent directories as needed
-v - Verbose (print each directory)

Examples:

$ mkdir project
$ mkdir -p parent/child/grandchild
$ ls
parent  project

`touch` - Create Empty File / Update Timestamp

Syntax: touch [options] <file>

Common Options:

-c - Don't create if doesn't exist
-d - Use specified date/time

Examples:

$ touch newfile.txt
$ ls
newfile.txt

`cp` - Copy Files/Directories

Syntax: cp [options] <source> <destination>

Common Options:

-r - Recursive (for directories)
-i - Interactive (prompt before overwrite)
-v - Verbose
-p - Preserve attributes

Examples:

$ cp file.txt backup.txt
$ cp -r src/ dst/

`mv` - Move/Rename Files

Syntax: mv [options] <source> <destination>

Common Options:

-i - Interactive (prompt before overwrite)
-v - Verbose

Examples:

$ mv oldname.txt newname.txt
$ mv file.txt /tmp/

`rm` - Remove Files/Directories

Syntax: rm [options] <file>

Common Options:

-r - Recursive (for directories)
-i - Interactive (prompt before removal)
-f - Force (ignore nonexistent files)

Warning: rm is permanent. Use with caution!

Examples:

$ rm file.txt
$ rm -r directory/

Text Processing Commands

`cat` - Concatenate and Display Files

Syntax: cat [options] <file>

Common Options:

-n - Number lines
-b - Number non-blank lines

Example:

$ cat -n file.txt
     1	First line
     2	Second line

`less` - Page Through Files

Syntax: less <file>

Navigation:

Space / f - Forward one page
b - Backward one page
g - Go to first line
G - Go to last line
/pattern - Search forward
q - Quit

`head` / `tail` - Display File Start/End

Syntax:

head [options] <file>
tail [options] <file>

Common Options:

-n N - Show N lines (default 10)
-f - Follow file (tail only, live updates)

Examples:

$ head -n 5 file.txt    # First 5 lines
$ tail -f /var/log/syslog  # Follow log file

`grep` - Print Matching Lines

Syntax: grep [options] <pattern> <file>

Common Options:

-i - Case insensitive
-r - Recursive directory search
-v - Invert match (show non-matching)
-n - Show line numbers
-c - Count matches

Examples:

$ grep "error" log.txt
ERROR: Connection failed

$ grep -r "TODO" src/
src/main.c: // TODO: Fix this

$ grep -n "main" file.txt
42: int main() {

`find` - Search for Files

Syntax: find [path] [options]

Common Tests:

-name <pattern> - Name matches pattern
-type <type> - File type (f=file, d=directory)
-size <n> - File size
-mtime <n> - Modified n days ago

Examples:

$ find . -name "*.txt"
./notes.txt
./todo.txt

$ find /var/log -type f -name "*.log"
/var/log/system.log

$ find . -type f -mtime -7    # Modified in last 7 days

Permission Commands

`chmod` - Change Mode (Permissions)

Syntax: chmod [options] <mode> <file>

Symbolic Mode:

chmod u+x file    # Add execute for user
chmod g-w file    # Remove write for group
chmod o=r file    # Set read-only for others
chmod a+rw file   # Add read-write for all

Numeric Mode:

chmod 755 file    # rwxr-xr-x
chmod 644 file    # rw-r--r--
chmod 777 file    # rwxrwxrwx (not recommended)

Permission Values:

4 = Read (r)
2 = Write (w)
1 = Execute (x)

`chown` - Change Owner

Syntax: chown [options] <user>[:<group>] <file>

Examples:

$ sudo chown john file.txt
$ sudo chown john:developers file.txt
$ sudo chown -R john:john directory/    # Recursive

`sudo` - Execute as Superuser

Syntax: sudo [options] <command>

Examples:

$ sudo apt install nginx
$ sudo systemctl restart ssh
$ sudo -u postgres psql    # Run as postgres user

Process Commands

`ps` - Process Status

Syntax: ps [options]

Common Options:

aux - Show all processes for all users
-u <user> - Show processes for user
-p <pid> - Show specific PID

Example:

$ ps aux
USER   PID  %CPU  %MEM  COMMAND
root     1   0.0   0.1  systemd
root   123   0.5   1.2  nginx

`top` / `htop` - Process Monitor

Interactive process viewers.

top keys:

q - Quit
k - Kill process
M - Sort by memory
P - Sort by CPU

`kill` - Send Signal to Process

Syntax: kill [options] <pid>

Common Signals:

SIGTERM (15) - Terminate gracefully
SIGKILL (9) - Force kill
SIGHUP (1) - Reload configuration

Examples:

$ kill 1234              # Graceful terminate
$ kill -9 1234           # Force kill
$ kill -HUP 1234         # Reload config

Package Management

Fedora (DNF)

Command	Description
`sudo dnf search <pkg>`	Search for package
`sudo dnf install <pkg>`	Install package
`sudo dnf remove <pkg>`	Remove package
`sudo dnf update`	Update all packages
`sudo dnf upgrade`	Upgrade distribution
`sudo dnf list installed`	List installed packages
`sudo dnf info <pkg>`	Show package info
`sudo dnf repolist`	List repositories

Debian (APT)

Command	Description
`sudo apt update`	Update package lists
`sudo apt upgrade`	Upgrade packages
`sudo apt install <pkg>`	Install package
`sudo apt remove <pkg>`	Remove package
`sudo apt purge <pkg>`	Remove with config
`sudo apt search <pkg>`	Search packages
`sudo apt show <pkg>`	Show package info
`sudo apt autoremove`	Remove unused packages

Systemd Commands

`systemctl` - Control systemd System

Syntax: systemctl [command] [unit]

Commands:

sudo systemctl start <service>     # Start service
sudo systemctl stop <service>      # Stop service
sudo systemctl restart <service>   # Restart service
sudo systemctl status <service>    # Show status
sudo systemctl enable <service>    # Enable at boot
sudo systemctl disable <service>   # Disable at boot
sudo systemctl reload <service>    # Reload config
systemctl list-units --type=service    # List services
systemctl --failed                   # List failed units

Examples:

$ sudo systemctl start nginx
$ sudo systemctl enable nginx
$ systemctl status nginx

`journalctl` - Query systemd Journal

Syntax: journalctl [options]

Common Options:

-u <unit> - Show logs for specific unit
-f - Follow logs (live)
-b - Show logs from current boot
-r - Show in reverse order
--since "time" - Show logs since time
--until "time" - Show logs until time

Examples:

$ journalctl -u nginx          # Nginx logs
$ journalctl -u nginx -f       # Follow nginx logs
$ journalctl -b -p err         # Errors from current boot
$ journalctl --since "1 hour ago"

Networking Commands

`ip` - Network Configuration

Show IP addresses:

$ ip addr show
$ ip a      # Short form

Show routes:

$ ip route show

`ping` - Test Network Connectivity

Syntax: ping [options] <host>

Options:

-c <count> - Send count packets
-i <interval> - Interval between packets

Example:

$ ping -c 4 google.com
PING google.com (142.250.185.46): 56 data bytes
64 bytes from 142.250.185.46: icmp_seq=0 ttl=54 time=12.3 ms

`ssh` - Secure Shell

Syntax: ssh [options] [user@]host [command]

Common Options:

-p <port> - Specify port
-i <identity> - Use identity file
-v - Verbose

Examples:

$ ssh user@server.com
$ ssh -p 2222 user@server.com
$ ssh user@server.com 'ls -la'

SSH Key Management:

$ ssh-keygen -t ed25519      # Generate key
$ ssh-copy-id user@host      # Copy public key

`scp` - Secure Copy

Syntax: scp [options] <source> <destination>

Examples:

$ scp file.txt user@host:/path/    # Upload
$ scp user@host:/path/file.txt .   # Download
$ scp -r directory/ user@host:/path/  # Recursive

Git Commands

Basic Git Commands

Command	Description
`git init`	Initialize new repository
`git clone <url>`	Clone existing repository
`git status`	Show working tree status
`git add <file>`	Stage file for commit
`git add .`	Stage all changes
`git commit -m "msg"`	Commit staged changes
`git log`	Show commit history
`git log --oneline`	Concise commit history
`git diff`	Show unstaged changes
`git diff --staged`	Show staged changes
`git branch`	List branches
`git branch <name>`	Create branch
`git checkout <branch>`	Switch branch
`git switch <branch>`	Switch branch (newer)
`git merge <branch>`	Merge branch into current
`git remote -v`	Show remotes
`git push`	Push to remote
`git pull`	Pull from remote
`git stash`	Stash changes
`git stash pop`	Apply stashed changes

Docker Commands

Basic Docker Commands

Command	Description
`docker pull <image>`	Pull image from registry
`docker run [options] <image>`	Run container
`docker ps`	List running containers
`docker ps -a`	List all containers
`docker stop <container>`	Stop container
`docker start <container>`	Start stopped container
`docker rm <container>`	Remove container
`docker rmi <image>`	Remove image
`docker images`	List images
`docker exec -it <container> bash`	Execute in container
`docker logs <container>`	Show container logs
`docker inspect <object>`	Show low-level info

Common Run Options

docker run -d              # Detached (background)
docker run -it             # Interactive + TTY
docker run -p 80:80        # Port mapping host:container
docker run -v /host:/container  # Volume mount
docker run --name myname   # Name container
docker run --rm           # Remove on exit

Examples

# Run nginx in background
docker run -d -p 80:80 --name web nginx

# Run Ubuntu interactively
docker run -it ubuntu bash

# Run with volume mount
docker run -v $(pwd):/app -w /app python:3.11 python script.py

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