Command Line Guide

Here we learn the command line - a text interface to computers. You input text commands to perform operations like creating or removing files, changing file permissions, and more.

On Mac, you can use the command line in Terminal.app.

Keyboard Shortcuts

Essential keyboard shortcuts for efficient terminal usage:

Basic Control

Shortcut	Description
Ctrl + C	Interrupt the current process
Ctrl + Z	Suspend the current process
Ctrl + D	Exit the shell
Ctrl + L	Clear the screen
Ctrl + S	Pause command output
Ctrl + Q	Resume output (after Ctrl+S)

Shortcut	Description
Ctrl + A	Move cursor to start of line
Ctrl + E	Move cursor to end of line
Ctrl + Left/Right	Move by word
Alt + B / Alt + F	Move backward/forward by word
Ctrl + T	Swap characters
Alt + T	Swap words

Cut, Copy, and Undo

Shortcut	Description
Ctrl + W	Cut the word before cursor
Ctrl + U	Cut everything before cursor
Ctrl + K	Cut everything after cursor
Ctrl + Y	Paste the last cut text
Ctrl + _	Undo

Command History

Shortcut	Description
↑ / ↓	Previous/next command
Ctrl + R	Search command history (reverse)
Ctrl + G	Exit history search

Special Symbols

Four special symbols represent important locations:

Symbol	Meaning	Example
`/`	root directory	`cd /`
`.`	current directory	`ls .`
`..`	parent directory	`cd ..`
`~`	home directory	`cd ~`

Basic Commands

pwd: Print working directory (show current location)

pwd
# /Users/username/Documents

cd: Change directory

cd ~/Documents        # Go to Documents
cd ..                 # Go up one level
cd -                  # Go to previous directory

ls: List files and directories

ls                    # List current directory
ls -l                 # Long format with details
ls -a                 # Show hidden files (starting with .)
ls -lh                # Human-readable file sizes
ls -ltr               # Sort by time, reverse order

Working with Directories

mkdir: Make directory

mkdir project                    # Create single directory
mkdir -p dir/subdir/subsubdir    # Create nested directories

rmdir: Remove empty directory

mkdir temp
rmdir temp                       # Only works if empty

rmdir -p: Remove directory hierarchy

mkdir -p dir/subdir/subsubdir
rmdir -p dir/subdir/subsubdir    # Removes all three

Working with Files

touch: Create empty file or update timestamp

touch file.txt                   # Create new file
touch existing.txt               # Update modification time

cat: Display file contents

cat file.txt                    # Show entire file
cat file1.txt file2.txt         # Show multiple files
cat > newfile.txt               # Create file with input (Ctrl+D to finish)

head / tail: Show beginning or end of file

head file.txt                   # First 10 lines (default)
head -n 5 file.txt              # First 5 lines
tail -f logfile.txt             # Follow file in real-time (useful for logs)

less / more: View files page by page

less largefile.txt              # Navigate with arrow keys, q to quit

rm: Remove files

rm file.txt                     # Remove file
rm -i file.txt                  # Confirm before removing
rm -v file.txt                  # Verbose (show what's removed)
rm -f file.txt                  # Force remove (no confirmation)

rm -r: Remove directories recursively

rm -r directory                  # Remove directory and contents
rm -rf directory                 # Force remove (CAUTION: no recovery!)

Warning

Be extremely careful with rm -rf - it permanently deletes without confirmation!

Copying and Moving

cp: Copy files or directories

cp source.txt dest.txt           # Copy file
cp -r dir1 dir2                  # Copy directory recursively
cp -i source.txt dest.txt        # Confirm before overwriting
cp file1 file2 file3 target/     # Copy multiple files to directory

Note

Copy behavior:

cp file1 file2 - This overwrite file2 with contents of file1.
cp -r dir1 dir2 - If dir2 exists, creates dir1 inside dir2. If dir2 doesn't exist, creates exact copy

mv: Move or rename files/directories

mv old.txt new.txt              # Rename file
mv file.txt directory/          # Move file to directory
mv dir1 dir2                    # Move/rename directory

File Information

file: Determine file type

file document.pdf
# document.pdf: PDF document, version 1.4

wc: Word count

wc file.txt                     # Lines, words, bytes
wc -l file.txt                  # Count lines only
wc -w file.txt                  # Count words only

du: Disk usage

du -h file.txt                  # Human-readable size
du -sh directory/               # Summary of directory size

Text Processing

echo: Print text or create files with content

echo "Hello World"              # Print to screen
echo $PATH                      # Print environment variable
echo "text" > file.txt          # Create/overwrite file
echo "more" >> file.txt         # Append to file

grep: Search for patterns in files

grep "word" file.txt            # Search for word
grep -i "Word" file.txt         # Case-insensitive search
grep -n "word" file.txt         # Show line numbers
grep -r "word" directory/       # Search recursively in directory
grep -v "word" file.txt         # Show lines NOT containing word
grep -c "word" file.txt         # Count matching lines
grep -w "word" file.txt         # Match whole word only
grep -l "word" *.txt            # List files containing word
grep -o "word" file.txt         # Show only matched parts
grep --color "word" file.txt    # Highlight matches in color

sed: Stream editor for text manipulation

sed 's/old/new/' file.txt         # Replace first occurrence per line
sed 's/old/new/g' file.txt        # Replace all occurrences
sed '2d' file.txt                 # Delete line 2
sed -i '' 's/old/new/g' file.txt  # Edit file in-place (Mac)

awk: Pattern scanning and processing

awk '{print $1}' file.txt       # Print first column
awk '{print $NF}' file.txt      # Print last column
awk '/pattern/ {print $0}' file.txt  # Print lines matching pattern

Permissions

Permissions control who can open, modify, or run a file or directory. You can set file permissions for the file owner, group, and others using a symbolic or numeric (octal) method with the chmod command.

Checking Current Permissions

You can view current permissions using ls -l:

ls -l file.txt
# -rwxr-xr--  1 user  group  1234 Aug  4 15:18 file.txt

The first 10 characters represent the file type and permissions:

The first character indicates the file type (- for regular file, d for directory).
The next 9 characters rwxr-xr-- represent permissions for owner, group, and others.
r = read, w = write, x = execute, - = no permission.
owner: rwx (read, write, execute)
group: r-x (read, execute)
others: r-- (read only)

Symbolic Permission Changes

You can change permissions using the following symbols and syntax with chmod:

Targets:

u = user (owner)
g = group
o = others
a = all (user + group + others)

Operations:

= = set exact permissions
+ = add permission
- = remove permission

Permission Types:

r = read the file's contents (for directories: list files).
w = modify the file (for directories: create/delete files).
x = run the file as a program (for directories: enter the directory / use cd).

Examples:

chmod u+x script.sh             # Make executable for owner
chmod g-w file.txt              # Remove write for group
chmod o-r secret.txt            # Remove read for others
chmod a+r file.txt              # Add read for all
chmod u=rwx,g=rx,o=r file.txt   # Set specific permissions

Numeric (Octal) Permission Changes (Advanced)

You can also set file permissions using a three-digit octal number, like chmod 755 file.txt. The three digits represent permissions for: [owner] [group] [others]. Each digit is a sum of the following values:

4 = read (r)
2 = write (w)
1 = execute (x)
0 = no permission

Common permission codes:

chmod 755 file.txt
# 7 (owner):  4+2+1 = read + write + execute
# 5 (group):  4+1   = read + execute
# 5 (others): 4+1   = read + execute

chmod 644 file.txt
# 6 (owner):  4+2 = read + write
# 4 (group):  4   = read only
# 4 (others): 4   = read only

chmod 777 file.txt              # Full permissions for everyone (rarely needed)
chmod 700 file.txt              # Private file (owner only)

chmod -R: Change permissions on directories and their contents recursively

chmod -R 755 directory/         # Apply to directory and all contents

Be careful — applying 755 recursively can make private files readable by everyone.

Permission Code Correspondence

Permission	Owner	Group	Others	Description
`700`	`rwx`	`---`	`---`	Private file (owner full access only)
`755`	`rwx`	`r-x`	`r-x`	Common for executable scripts
`644`	`rw-`	`r--`	`r--`	Common for documents/config files
`600`	`rw-`	`---`	`---`	Read/write for owner only
`777`	`rwx`	`rwx`	`rwx`	Full access for everyone (rarely safe)

Changing owner and group

chown: Change file owner:

chown user file.txt             # Change owner
chown user:group file.txt       # Change owner and group
chown -R user directory/        # Recursive

Finding Files

find: Search for files and directories

# Basic syntax
find [path] [options] [expression]

# Find by name
find . -name "*.txt"                 # Find all .txt files
find . -iname "*.TXT"                # Case-insensitive

# Find by type
find . -type f                       # Find files only
find . -type d                       # Find directories only

# Find by time (days)
find . -mtime -7                     # Modified in last 7 days
find . -atime +30                    # Accessed more than 30 days ago

# Find by size
find . -size +10M                    # Larger than 10MB
find . -size -100k                   # Smaller than 100KB
find . -size +1M -size -10M          # Between 1MB and 10MB

# Combine conditions
find . -name "*.log" -mtime +30      # Old log files
find . -type f -empty                # Empty files

# Execute commands on results
find . -name "*.tmp" -delete         # Delete all .tmp files
find . -name "*.txt" -exec rm {} \;  # Remove each file
find . -name "*.txt" -exec rm {} +   # Remove all at once (faster)

locate: Fast file search using database

locate filename                 # Quick search (may need: sudo updatedb)

which: Find command location

which python                    # Shows path to python executable

whereis: Find binary, source, and manual pages

whereis python                  # Shows multiple related paths

Wildcards and Pattern Matching

Shell Globbing

Basic wildcards:

*                               # Matches zero or more characters
?                               # Matches exactly one character
[abc]                           # Matches any character in set
[a-z]                           # Matches any character in range
[!abc]                          # Matches any character NOT in set

Examples:

ls *.txt                        # All .txt files
ls file?.txt                    # file1.txt, fileA.txt, etc.
ls [abc]*.txt                   # Files starting with a, b, or c
ls [0-9][0-9].txt               # Two-digit numbered files
rm temp*                        # Remove all files starting with temp

Extended Pattern Matching

Enable with: shopt -s extglob

*(pattern)                      # Matches 0 or more times
+(pattern)                      # Matches 1 or more times
?(pattern)                      # Matches 0 or 1 time
@(pattern)                      # Matches exactly once
!(pattern)                      # Matches anything except pattern

Examples:

# Setup
shopt -s extglob
ls
# file1 file2 file3 file11 file123

# Examples
ls !(file1)                     # Everything except file1
ls !(file1|file2)               # Everything except file1 and file2
ls file[0-9]                    # file1 file2 file3 file4
ls file+([0-9])                 # file1 file2 file3 file4 file11 file123

Brace Expansion

Generate arbitrary strings:

echo {1,2,3}                     # 1 2 3
echo file{1..5}.txt              # file1.txt file2.txt ... file5.txt
echo {a..z}                      # a b c ... z

# Combine patterns
echo {A,B}{1,2}                  # A1 A2 B1 B2

# Practical examples
mkdir -p project/{src,test,docs}
touch file{1..10}.txt
cp file.txt{,.backup}            # Quick backup: file.txt.backup
mv /path/{foo,bar,baz}.txt dir/  # Move multiple specific files

Pipes and Redirection

Understanding Streams

Unix has three standard streams:

stdin (0) - Standard input (keyboard)
stdout (1) - Standard output (screen)
stderr (2) - Standard error (screen)

Redirection Operators

command > file                  # Redirect stdout to file (overwrite)
command >> file                 # Redirect stdout to file (append)
command 2> file                 # Redirect stderr to file
command &> file                 # Redirect both stdout and stderr
command > file 2>&1             # Redirect stdout and stderr (alternative)
command < file                  # Read stdin from file
command 2>/dev/null             # Discard error messages
command &>/dev/null             # Discard all output

Examples:

ls -l > listing.txt               # Save output to file
echo "log entry" >> log.txt       # Append to log file
find / -name "*.txt" 2>/dev/null  # Search without permission errors

Pipes

Chain commands by sending output of one as input to another:

command1 | command2                  # Pipe stdout of command1 to command2

# Examples
ls -l | grep ".txt"                  # List only .txt files
cat file.txt | wc -l                 # Count lines in file
history | tail -20                   # Show last 20 commands
ps aux | grep python                 # Find Python processes
cat access.log | grep "404" | wc -l  # Count 404 errors

Useful pipe combinations:

# Sort and remove duplicates
cat file.txt | sort | uniq

# Count unique occurrences
cat file.txt | sort | uniq -c

# Find largest files
du -h | sort -rh | head -10

# Monitor system processes
ps aux | grep -i chrome | wc -l

Combining Commands

Sequential execution (;) - Run commands one after another

command1 ; command2             # command2 runs regardless of command1's success
cd /tmp ; ls ; pwd

Background execution (&) - Run command in background

command1 & command2             # Both run simultaneously
long_process & echo "Started"

Conditional AND (&&) - Run next command only if previous succeeds

command1 && command2            # command2 runs only if command1 succeeds
mkdir project && cd project
make && make test && make install

Conditional OR (||) - Run next command only if previous fails

command1 || command2            # command2 runs only if command1 fails
cd /tmp || echo "Failed to change directory"

Command substitution ($(...)) - Use output of command as argument

echo "Today is $(date)"
touch file_$(date +%Y%m%d).txt
ls $(dirname $(which python))   # Nested commands

xargs: Build and execute commands from standard input

# Basic usage
echo "file1 file2 file3" | xargs rm

# Common patterns
find . -name "*.tmp" | xargs rm              # Remove found files
find . -type f -name "*.txt" | xargs grep "pattern"  # Search in found files

# Handle filenames with spaces
find . -name "*.txt" -print0 | xargs -0 rm   # Use null separator

# Control execution
ls *.txt | xargs -n 1 echo "Processing:"     # One argument at a time
ls *.txt | xargs -I {} cp {} backup/         # Use placeholder

Directory Visualization

tree: Display directory structure in tree format

Install first (if needed):

brew install tree               # macOS

Usage:

tree                            # Show full tree
tree -L 2                       # Limit depth to 2 levels
tree -d                         # Directories only
tree -a                         # Include hidden files
tree -I 'node_modules|*.pyc'    # Ignore patterns
tree --dirsfirst                # List directories first
tree -h                         # Human-readable sizes

# Example output
project/
├── src/
│   ├── main.py
│   └── utils.py
├── tests/
│   └── test_main.py
└── README.md

Symbolic Links

ln -s: Create symbolic links (symlinks). A symbolic link is a file that points to another file or directory. It's like a shortcut or alias.

ln -s /path/to/original /path/to/link

# Examples
ln -s ~/Documents/project ~/Desktop/project-link
ln -s /usr/local/bin/python3 ~/bin/python

# Verify link
ls -l link_file
# lrwxr-xr-x  1 user  group  12 Aug  4 15:18 link_file -> original_file

Useful ln options:

ln -sf original link            # Force create (overwrite existing)
ln -s ~/Documents/notes.txt .   # Create link in current directory

Key points:

Symlinks can point to files or directories
If original is deleted, symlink breaks
Relative vs absolute paths affect portability

Note

macOS Finder Aliases vs Symlinks:

Finder Aliases: Remain valid if original file moves
Symlinks: Break if original file moves
Compatibility: Symlinks work in terminal (cd symlink), Finder aliases don't

Hard links (less common):

ln original hardlink            # Create hard link (no -s flag)

Both files point to same data
Deleting one doesn't affect the other
Cannot link directories or across filesystems

Environment and Configuration

Environment variables:

echo $HOME                      # Show home directory
echo $PATH                      # Show executable search paths
echo $USER                      # Show current username

# Set variables
export VAR_NAME="value"         # Set for current session

Aliases:

alias ll='ls -lah'              # Create alias
alias ..='cd ..'
unalias ll                      # Remove alias

# Make permanent: add to ~/.zshrc or ~/.bashrc

View command history:

history                         # Show all commands
history 20                      # Show last 20 commands
!123                            # Run command #123 from history
!!                              # Repeat last command
!$                              # Last argument of previous command

Process Management

View processes:

ps                              # Current shell processes
ps aux                          # All processes
ps aux | grep python            # Find specific processes
top                             # Interactive process viewer
htop                            # Better process viewer (may need install)

Control processes:

command &                       # Run in background
jobs                            # List background jobs
fg %1                           # Bring job 1 to foreground
bg %1                           # Resume job 1 in background
kill PID                        # Kill process by ID
kill -9 PID                     # Force kill
killall process_name            # Kill all processes by name

Tips and Best Practices

1. Safety First

# Always use -i for interactive confirmation when deleting
alias rm='rm -i'
alias mv='mv -i'
alias cp='cp -i'

# Make backups before destructive operations
cp important.txt{,.backup}

2. Efficiency Tricks

# Quick directory navigation
cd -                           # Go to previous directory
pushd /path && popd            # Save and return to directories

# Quick file editing
open -e file.txt               # macOS: open in TextEdit
nano file.txt                  # Simple terminal editor
vim file.txt                   # Powerful terminal editor

3. Information Gathering

man command                    # Read manual page
command --help                 # Quick help
which command                  # Find command location
type command                   # Show command type (alias/function/builtin)

Practice Exercises

Try these exercises to build your command line skills:

# 1. Create a project structure
mkdir -p myproject/{src,tests,docs}
touch myproject/src/{main.py,utils.py}
touch myproject/README.md

# 2. Find and count
find . -name "*.py" | wc -l

# 3. Search and replace
find . -name "*.txt" -exec sed -i '' 's/old/new/g' {} +

# 4. Monitor log files
tail -f /var/log/system.log | grep ERROR

Quick Reference

# Navigation
pwd, cd, ls

# File Operations  
touch, cat, cp, mv, rm, mkdir, rmdir

# Text Processing
grep, sed, awk, sort, uniq, wc

# Search
find, locate, which

# Permissions
chmod, chown

# Pipes & Redirection
|, >, >>, <, 2>, &>

# Process Control
ps, top, kill, jobs, fg, bg

# Information
man, --help, which, type

Command Line Guide

Table of Contents

Command Line Guide

Keyboard Shortcuts

Basic Control

Navigation & Editing

Cut, Copy, and Undo

Command History

Special Symbols

Basic Commands

Navigation and Information

Working with Directories

Working with Files

Copying and Moving

File Information

Text Processing

Permissions

Checking Current Permissions

Symbolic Permission Changes

Numeric (Octal) Permission Changes (Advanced)

Permission Code Correspondence

Changing owner and group

Finding Files

Wildcards and Pattern Matching

Shell Globbing

Extended Pattern Matching

Brace Expansion

Pipes and Redirection

Understanding Streams

Redirection Operators

Pipes

Combining Commands

Directory Visualization

Symbolic Links

Environment and Configuration

Process Management

Tips and Best Practices

1. Safety First

2. Efficiency Tricks

3. Information Gathering

Practice Exercises

Quick Reference