Bash Command Reference

What is Bash?

Bash (Bourne Again Shell) is a command-line interpreter and scripting language used for: - System automation - File management - Process control - Task scheduling

Similar to Python but specialized for system operations and shell scripting.

Basic Command Syntax

command [options] [arguments]

Components: - command: The operation to perform (e.g., ls, cd, mkdir) - options: Flags that modify command behavior (prefixed with - or --) - arguments: What the command acts on (files, directories, strings)

Option Formats

Single Dash (-) - Short Options

Single-letter flags that can be combined:

ls -l           # Long listing format
ls -a           # Show all files (including hidden)
ls -la          # Combined: long format + all files
ls -lah         # Long + all + human-readable sizes

Double Dash (--) - Long Options

Multi-character descriptive flags:

ls --all                    # Show all files
tar --exclude="*.log"       # Exclude log files
grep --ignore-case "text"   # Case-insensitive search

End of Options Marker (--)

Explicitly marks the end of options:

rm -- -filename.txt    # Delete file named "-filename.txt"
grep -- "--help" file  # Search for the literal string "--help"

Change Directory

pwd                     # Print working directory (show current location)
cd /path/to/directory   # Go to specific directory
cd ~                    # Go to home directory
cd                      # Go to home (same as ~)
cd ..                   # Go up one level
cd ../..                # Go up two levels
cd -                    # Go to previous directory
cd /                    # Go to root directory

Examples:

cd ~/Documents          # Go to Documents in home
cd ../../projects       # Go up 2 levels, then into projects
cd -                    # Toggle between last two directories

Listing Files and Directories

Basic Listing

ls                      # List files in current directory
ls /path/to/dir         # List files in specific directory

Common Options

Option Description Example
-l Long listing format (permissions, owner, size, date) ls -l
-a Show all files (including hidden . files) ls -a
-h Human-readable file sizes (KB, MB, GB) ls -lh
-t Sort by modification time (newest first) ls -lt
-r Reverse sort order ls -lr
-R List subdirectories recursively ls -R
-S Sort by file size (largest first) ls -lS
-1 List one file per line ls -1
-d List directories themselves, not contents ls -ld */

Practical Examples

# Detailed view with human-readable sizes
ls -lh

# All files including hidden, long format, human-readable
ls -lah

# Sort by modification time, newest first
ls -lt

# Sort by size, largest first
ls -lS

# Show only directories
ls -d */

# Recursive listing of all files
ls -R

# Combine: all files, long format, human-readable, sorted by time
ls -laht

Creating Files and Directories

Create Directories

# Create single directory
mkdir newdirectory

# Create nested directories
mkdir -p parent/child/grandchild

# Create multiple directories
mkdir dir1 dir2 dir3

# Create with specific permissions
mkdir -m 755 public_folder

Create Files

# Create empty file
touch filename.txt

# Create multiple files
touch file1.txt file2.txt file3.txt

# Update timestamp of existing file
touch existing_file.txt

Removing Files and Directories

Remove Files

# Remove single file
rm filename.txt

# Remove multiple files
rm file1.txt file2.txt

# Remove all .txt files
rm *.txt

Remove Directories

# Remove empty directory
rmdir emptydirectory

# Remove directory and all contents
rm -r directoryname

# Force remove without prompts
rm -rf directoryname

Options Explained

Option Description
-r Recursive - delete directory and everything inside
-i Interactive - ask before deleting each file
-f Force - delete without asking, ignore nonexistent files
-v Verbose - show files being removed

Safety Examples

# Interactive deletion (safe)
rm -i important_file.txt

# Show what's being deleted
rm -v *.log

# Careful deletion with confirmation
rm -ri old_project/

# Dangerous: force delete everything (use with caution!)
rm -rf folder/          # ⚠️ No confirmation, permanent!

⚠️ Warning: rm -rf is irreversible. Always double-check before using!

Copying and Moving

Copy Files and Directories

# Copy file
cp sourcefile.txt destination.txt

# Copy to directory
cp file.txt /path/to/directory/

# Copy multiple files to directory
cp file1.txt file2.txt /destination/

# Copy directory and all contents
cp -r sourcedirectory/ destinationdirectory/

Copy Options

Option Description
-r Recursive - copy directories
-i Interactive - prompt before overwrite
-v Verbose - show files being copied
-u Update - copy only newer files
-p Preserve file attributes (permissions, timestamps)

Examples:

# Copy with confirmation before overwrite
cp -i source.txt destination.txt

# Copy entire directory
cp -r project/ backup_project/

# Copy preserving permissions and timestamps
cp -rp source/ destination/

Move and Rename

# Rename file
mv oldname.txt newname.txt

# Move file to directory
mv file.txt /path/to/directory/

# Move multiple files
mv file1.txt file2.txt /destination/

# Move and rename simultaneously
mv old_project/file.txt new_project/renamed_file.txt

# Move directory
mv old_directory/ new_location/

Examples:

# Rename with confirmation
mv -i old.txt new.txt

# Move with verbose output
mv -v *.txt documents/

# Move all files from one directory to another
mv source_dir/* destination_dir/

Viewing and Editing Files

View File Contents

# Display entire file
cat filename.txt

# Display with line numbers
cat -n filename.txt

# View file page by page (forward only)
more filename.txt

# View file with navigation (forward and backward)
less filename.txt

# Display first 10 lines
head filename.txt

# Display first 20 lines
head -n 20 filename.txt

# Display last 10 lines
tail filename.txt

# Display last 20 lines
tail -n 20 filename.txt

# Follow file updates in real-time (useful for logs)
tail -f logfile.txt

Edit Files

# Edit with nano (beginner-friendly)
nano filename.txt

# Edit with vim (advanced)
vim filename.txt

# Edit with vi (basic)
vi filename.txt

Nano shortcuts: - Ctrl + O - Save - Ctrl + X - Exit - Ctrl + K - Cut line - Ctrl + U - Paste

Searching

Search Inside Files (grep)

# Search for text in file
grep "search_term" filename.txt

# Case-insensitive search
grep -i "search_term" filename.txt

# Search recursively in directory
grep -r "search_term" /path/to/directory/

# Search and show line numbers
grep -n "search_term" filename.txt

# Search for whole word only
grep -w "word" filename.txt

# Invert match (show lines that DON'T match)
grep -v "exclude_this" filename.txt

# Count matching lines
grep -c "pattern" filename.txt

Grep Options

Option Description
-i Ignore case (case-insensitive)
-r Recursive search in directories
-n Show line numbers
-v Invert match (exclude matching lines)
-w Match whole words only
-c Count matching lines
-l Show only filenames with matches
-A 3 Show 3 lines after match
-B 3 Show 3 lines before match
-C 3 Show 3 lines before and after match

Examples:

# Find all Python files containing "import"
grep -r "import" *.py

# Find TODO comments with line numbers
grep -n "TODO" src/*.js

# Find files containing "error" (case-insensitive)
grep -ril "error" logs/

# Search with context (3 lines before and after)
grep -C 3 "exception" app.log

Find Files (find)

# Find files by name
find /path -name "filename.txt"

# Find files by pattern
find . -name "*.txt"

# Find directories
find . -type d -name "dirname"

# Find files modified in last 7 days
find . -type f -mtime -7

# Find files larger than 100MB
find . -type f -size +100M

# Find and delete empty files
find . -type f -empty -delete

File Permissions

View Permissions

ls -l filename.txt
# Output: -rw-r--r-- 1 user group 1234 Jan 1 12:00 filename.txt

Permission format: -rw-r--r-- - First character: file type (- = file, d = directory) - Next 9 characters: permissions in groups of 3 - Owner (user): rw- (read, write, no execute) - Group: r-- (read only) - Others: r-- (read only)

Change Permissions (chmod)

# Numeric method (octal)
chmod 755 script.sh     # rwxr-xr-x
chmod 644 file.txt      # rw-r--r--
chmod 600 private.key   # rw-------

# Symbolic method
chmod +x script.sh      # Add execute permission
chmod -w file.txt       # Remove write permission
chmod u+x script.sh     # User: add execute
chmod g-w file.txt      # Group: remove write
chmod o-r secret.txt    # Others: remove read

Common permission values: - 755 - rwxr-xr-x - Executable files (owner can modify) - 644 - rw-r--r-- - Regular files (owner can modify) - 600 - rw------- - Private files (only owner can access) - 777 - rwxrwxrwx - Dangerous! (everyone has full access)

Input/Output Redirection

Output Redirection

# Redirect output to file (overwrite)
command > output.txt

# Append output to file
command >> output.txt

# Redirect errors to file
command 2> errors.txt

# Redirect both output and errors
command > output.txt 2>&1
command &> output.txt           # Shorter syntax

Input Redirection

# Read input from file
command < input.txt

# Pipe output to another command
cat file.txt | grep "pattern"
ls -l | grep ".txt"

Useful Tips and Shortcuts

Command History

history                 # Show command history
!123                    # Run command #123 from history
!!                      # Run last command
!grep                   # Run last command starting with "grep"
Ctrl + R                # Search command history (interactive)

Keyboard Shortcuts

Shortcut Action
Ctrl + C Cancel current command
Ctrl + D Exit shell (logout)
Ctrl + L Clear screen (same as clear)
Ctrl + A Move to beginning of line
Ctrl + E Move to end of line
Ctrl + U Delete from cursor to beginning
Ctrl + K Delete from cursor to end
Tab Auto-complete file/directory names
↑ / ↓ Navigate command history

Wildcards

*                       # Match any characters
?                       # Match single character
[abc]                   # Match a, b, or c
[0-9]                   # Match any digit

# Examples
ls *.txt                # All .txt files
ls file?.txt            # file1.txt, file2.txt, etc.
ls [abc]*.txt           # Files starting with a, b, or c

Quick Reference

pwd         # Current directory
cd dir      # Change directory
cd ..       # Up one level
cd -        # Previous directory

Files

ls -lah     # List all files, detailed, human-readable
mkdir dir   # Create directory
touch file  # Create file
rm file     # Remove file
rm -rf dir  # Remove directory
cp src dst  # Copy
mv old new  # Move/rename

grep "text" file        # Search in file
grep -r "text" dir/     # Search in directory
find . -name "*.txt"    # Find files

View

cat file        # Display file
head file       # First 10 lines
tail file       # Last 10 lines
tail -f file    # Follow file updates
less file       # Page through file

Safety Tips

  1. Always double-check before using rm -rf - it's irreversible!
  2. Use -i flag for confirmations when deleting/overwriting
  3. Test commands with echo first to see what will happen
  4. Make backups before bulk operations
  5. Use ls before rm to verify what you're deleting
# Safe practice: preview before deleting
ls *.log                # Preview files
rm -i *.log             # Delete with confirmation

Happy Bash scripting! 🐚💻