chapter_00005.md

Working with Files in Bash

File Test Operators

Use test operators to verify file properties. Always quote variables to handle spaces in filenames.

Common Operators:

Operator	Description
-e	File exists
-f	Regular file (not directory)
-d	Directory
-s	File exists and is not empty
-r	Readable by current user
-w	Writable by current user
-x	Executable by current user
-L	Symbolic link
-O	Owned by current user

Improved Example:

#!/bin/bash
file="example.txt"

if [ -e "$file" ]; then
  echo "File exists"
  if [ -s "$file" ]; then
    echo "File is non-empty"
  fi
else
  echo "File does not exist" >&2
  exit 1
fi

Reading and Writing Files

Best Practices:

• Use printf for more reliable output formatting
• Prefer $(< file) for efficient file reading
• Always check write permissions before modifying files

Enhanced Examples:

# Safe write with error checking
if [ -w . ]; then
  printf "%s\n" "Hello World" "Second line" > example.txt
else
  echo "No write permission in current directory" >&2
  exit 1
fi

# Read file into variable (alternative to cat)
content=$(< example.txt)
echo "$content"

# Process file line-by-line
while IFS= read -r line; do
  echo "Processing: $line"
done < example.txt

File Permissions

Understand permission modes:

• Symbolic: u+r (user read), g-w (group write remove)
• Octal: 755 (rwxr-xr-x), 644 (rw-r--r--)

Practical Examples:

# Set secure permissions
chmod 644 config.txt  # Owner: read/write, Others: read-only

# Make executable and verify
chmod u+x script.sh
[ -x script.sh ] && ./script.sh

Directory Management

Key Commands:

• mkdir -p: Create nested directories
• rm -rf: Remove directories recursively (use with caution)
• find: Advanced directory operations

Improved Examples:

# Create nested directory structure
mkdir -p project/{src,dist,test}

# Safe directory removal
dir_to_remove="temp/"
if [ -d "$dir_to_remove" ]; then
  rm -rf "$dir_to_remove"
  echo "Directory removed"
fi

File Handling Best Practices

1. Always quote file path variables
2. Verify operations with -i flags (e.g., rm -i)
3. Use mktemp for temporary files
4. Prefer absolute paths in critical operations

Enhanced Exercise

Create a script that:

1. Accepts a filename as an argument
2. Checks if it's a regular file and readable
3. Displays contents with line numbers
4. Handles edge cases (missing arguments, special files)

Improved Solution:

#!/bin/bash
# File inspection script

# Verify argument provided
if [ $# -ne 1 ]; then
  echo "Usage: $0 <filename>" >&2
  exit 1
fi

filename="$1"

# Perform safety checks
if [ ! -e "$filename" ]; then
  echo "Error: File '$filename' does not exist" >&2
  exit 2
elif [ ! -f "$filename" ]; then
  echo "Error: '$filename' is not a regular file" >&2
  exit 3
elif [ ! -r "$filename" ]; then
  echo "Error: No read permission for '$filename'" >&2
  exit 4
fi

# Display contents with line numbers
echo "Contents of $filename:"
nl -ba "$filename"

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