Command Line Basics

# Where am I?
pwd
# /home/alice

# What's in this directory?
ls
ls -la          # detailed list including hidden files
ls -lh          # human-readable file sizes

# Change directory
cd Documents           # go into Documents
cd ..                  # go up one level
cd ~                   # go to home directory
cd /                   # go to root
cd -                   # go to previous directory

# Create directories
mkdir projects
mkdir -p projects/my-app/src    # create nested directories

# Create files
touch index.html                # create empty file
echo "hello" > file.txt         # create file with content

# Copy
cp file.txt backup.txt          # copy file
cp -r folder/ folder-backup/    # copy directory recursively

# Move / rename
mv old-name.txt new-name.txt    # rename
mv file.txt Documents/          # move to another directory

# Delete
rm file.txt                     # delete file
rm -r folder/                   # delete directory and contents
rm -i file.txt                  # ask for confirmation

# Display entire file
cat file.txt

# Display with line numbers
cat -n file.txt

# First/last lines
head -20 file.txt       # first 20 lines
tail -20 file.txt       # last 20 lines
tail -f logfile.log     # follow (watch for new lines in real-time)

# Page through a long file
less file.txt           # scroll with arrows, q to quit

# Word/line/character count
wc file.txt             # lines, words, characters
wc -l file.txt          # just line count

# Find by name
find . -name "*.js"                    # all .js files in current directory tree
find /home -name "config.yml"          # search from /home

# Find by type
find . -type d -name "node_modules"    # directories only
find . -type f -name "*.log"           # files only

# Find by modification time
find . -mtime -7 -name "*.md"         # modified in last 7 days

# Search for text in files
grep "TODO" *.js                       # search in all .js files
grep -r "import" src/                  # recursive search in directory
grep -rn "error" logs/                 # show line numbers
grep -ri "password" .                  # case-insensitive

# Invert match (lines that DON'T contain pattern)
grep -v "debug" app.log

# Count matches
grep -c "error" app.log

# Find the 10 largest files
ls -lS | head -10

# Count how many .js files exist
find . -name "*.js" | wc -l

# Search command history
history | grep "docker"

# Sort and deduplicate
cat names.txt | sort | uniq

# Find processes using a port
lsof -i :3000 | grep LISTEN

# Write output to file (overwrite)
echo "hello" > file.txt
ls -la > directory-listing.txt

# Append to file
echo "another line" >> file.txt

# Redirect errors
command 2> errors.log

# Redirect both stdout and stderr
command > output.log 2>&1

# See running processes
ps aux                          # all processes
ps aux | grep node              # filter for node processes

# Real-time process monitor
top                             # basic
htop                            # better (install with apt/brew)

# Kill a process
kill 12345                      # graceful shutdown (SIGTERM)
kill -9 12345                   # force kill (SIGKILL)
killall node                    # kill all processes named "node"

# Run in background
long-command &                  # run in background
jobs                            # list background jobs
fg %1                           # bring job 1 to foreground

# View permissions
ls -la
# -rw-r--r--  1 alice staff  1024 May 18 14:00 file.txt
# drwxr-xr-x  3 alice staff    96 May 18 14:00 folder/

# Change permissions
chmod 755 script.sh             # rwxr-xr-x (owner: all, others: read+execute)
chmod +x script.sh              # add execute permission
chmod 644 file.txt              # rw-r--r-- (owner: read+write, others: read)

# Change ownership
chown alice:staff file.txt

# View all
env
printenv

# View one
echo $HOME
echo $PATH

# Set temporarily (current session only)
export API_KEY="abc123"

# Set permanently (add to ~/.bashrc or ~/.zshrc)
echo 'export API_KEY="abc123"' >> ~/.zshrc
source ~/.zshrc                 # reload without restarting terminal

# Disk usage
df -h                           # filesystem usage
du -sh *                        # size of each item in current directory
du -sh node_modules/            # size of a specific directory

# Network
curl https://api.example.com    # make HTTP requests
curl -s https://api.example.com | jq .   # pretty-print JSON
ping google.com                 # test connectivity
wget https://example.com/file   # download a file

# Archives
tar -czf archive.tar.gz folder/    # create compressed archive
tar -xzf archive.tar.gz            # extract archive
zip -r archive.zip folder/         # create zip
unzip archive.zip                  # extract zip

# System info
uname -a                        # OS info
whoami                          # current user
date                            # current date/time
uptime                          # how long system has been running

# Shortcuts
alias ll="ls -la"
alias gs="git status"
alias dc="docker compose"
alias ..="cd .."
alias ...="cd ../.."

# Safety nets
alias rm="rm -i"
alias mv="mv -i"