File Manipulation

File manipulation is at the heart of shell scripting. Whether you're organizing files, setting permissions, or finding specific files, Bash provides powerful commands for every task. In this lesson, you'll master cp, mv, rm, chmod, and the incredibly versatile find command!

Basic File Operations: cp, mv, rm

The fundamental file manipulation commands let you copy, move, rename, and delete files and directories.

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File Permissions with chmod

Unix file permissions control who can read, write, and execute files. The chmod command lets you modify these permissions.

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Understanding Permission Numbers

Finding Files with find

The find command is incredibly powerful for locating files based on name, type, size, time, permissions, and more.

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Essential find Options

# Find by name
find /path -name "*.txt"           # Case-sensitive
find /path -iname "*.TXT"          # Case-insensitive

# Find by type
find /path -type f                 # Files only
find /path -type d                 # Directories only
find /path -type l                 # Symbolic links

# Find by time
find /path -mtime -7               # Modified within 7 days
find /path -mtime +30              # Modified more than 30 days ago
find /path -mmin -60               # Modified within 60 minutes

# Find by size
find /path -size +100M             # Larger than 100MB
find /path -size -1k               # Smaller than 1KB

# Find by permissions
find /path -perm 755               # Exact permissions
find /path -perm -u+x              # At least user executable

# Execute command on results
find /path -name "*.log" -delete   # Delete matching files
find /path -name "*.sh" -exec chmod +x {} \;  # Make executable
find /path -type f -exec grep -l "pattern" {} \;  # Search in files

find /path -name "*.txt" | xargs grep "pattern"
find /path -name "*.log" -print0 | xargs -0 rm -f  # Handle spaces in names

Common Mistakes

# Wrong - can't copy directory without -r
cp mydir newdir        # Error!

# Correct
cp -r mydir newdir

# EXTREMELY DANGEROUS!
rm -rf /                  # Deletes everything
rm -rf $VAR/*            # If VAR is empty, becomes rm -rf /*

# Safer patterns
rm -rf "${VAR:?Error: VAR not set}/"*  # Fails if VAR is empty
[[ -n "$VAR" ]] && rm -rf "$VAR"/*

# Wrong - gives everyone write access
chmod 777 sensitive_file.txt  # DANGEROUS!

# Correct - restrictive permissions
chmod 600 sensitive_file.txt  # Owner only
chmod 644 public_file.txt     # Owner write, others read

# Wrong - shell expands * before find sees it
find /path -name *.txt

# Correct - quotes prevent shell expansion
find /path -name "*.txt"

Exercise: File Organizer Script

#!/bin/bash
# File Organizer Script

source_dir="/tmp/organize_test"
mkdir -p "$source_dir"

# Create test files
touch "$source_dir"/{file1.txt,file2.txt,app.log,error.log,script.sh,deploy.sh}

echo "=== File Organizer ==="
echo "Source: $source_dir"
echo ""

# Create category directories
mkdir -p "$source_dir"/{text_files,log_files,scripts}

# Move and count files
txt_count=0
log_count=0
sh_count=0

# Move .txt files
for f in "$source_dir"/*.txt 2>/dev/null; do
    [[ -f "$f" ]] || continue
    mv "$f" "$source_dir/text_files/"
    chmod 644 "$source_dir/text_files/$(basename "$f")"
    ((txt_count++))
done

# Move .log files
for f in "$source_dir"/*.log 2>/dev/null; do
    [[ -f "$f" ]] || continue
    mv "$f" "$source_dir/log_files/"
    chmod 644 "$source_dir/log_files/$(basename "$f")"
    ((log_count++))
done

# Move .sh files
for f in "$source_dir"/*.sh 2>/dev/null; do
    [[ -f "$f" ]] || continue
    mv "$f" "$source_dir/scripts/"
    chmod 755 "$source_dir/scripts/$(basename "$f")"
    ((sh_count++))
done

echo "Files organized:"
echo "  Text files: $txt_count"
echo "  Log files: $log_count"
echo "  Scripts: $sh_count"
echo ""
echo "Directory structure:"
find "$source_dir" -type f

# Cleanup
rm -rf "$source_dir"

Summary

What's Next?

Now let's explore File Testing in more depth. You'll learn advanced techniques for checking file properties, comparing files, and making your scripts robust with proper file validation!