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File Testing: Check Before You Use

Before using files, check if they exist. Check permissions. Check type.

Here's the thing: File tests prevent errors. Use them. Your scripts will be more reliable.

Basic File Tests​

Existence and Type​

[ -e file ]                      # File exists (any type)
[ -f file ]                      # Regular file
[ -d dir ]                       # Directory
[ -L file ]                      # Symbolic link

My take: -f for files. -d for directories. -e for anything. Use -f and -d most.

Permissions​

[ -r file ]                      # Readable
[ -w file ]                      # Writable
[ -x file ]                      # Executable
[ -s file ]                      # Non-empty (size > 0)

My take: Check permissions before using files. Prevents errors.

Using File Tests​

Basic Check​

if [ -f "$file" ]; then
    echo "File exists"
    process_file "$file"
else
    echo "File not found"
fi

My take: Always check before using. Prevents errors.

Check Multiple Conditions​

if [ -f "$file" ] && [ -r "$file" ]; then
    echo "File exists and is readable"
fi

My take: Combine tests. Check multiple conditions.

Common Patterns​

Check Before Processing​

if [ ! -f "$file" ]; then
    echo "Error: File not found: $file"
    exit 1
fi

process_file "$file"

Check Directory​

if [ ! -d "$dir" ]; then
    mkdir -p "$dir"
fi

Check Permissions​

if [ ! -r "$file" ]; then
    echo "Error: Cannot read $file"
    exit 1
fi

Common Mistakes (I've Made These)​

  1. Not checking existence: Files might not exist. Check first.

  2. Forgetting quotes: [ -f $file ] breaks with spaces. Use [ -f "$file" ].

  3. Wrong test: Using -f for directories. Use -d for directories.

  4. Not checking permissions: Files might not be readable. Check permissions.

  5. Assuming files exist: Don't assume. Check. Always.

Real-World Examples​

Process File if Exists​

if [ -f "$config_file" ]; then
    source "$config_file"
else
    echo "Using default configuration"
fi

Create Directory if Missing​

if [ ! -d "$backup_dir" ]; then
    mkdir -p "$backup_dir"
fi

Check Before Backup​

if [ -f "$file" ] && [ -r "$file" ]; then
    cp "$file" "${file}.backup"
else
    echo "Cannot backup $file"
fi

What's Next?​

Now that you can test files, scripts are more reliable. Or learn about File Reading to read file contents.

Personal note: When I started, I'd assume files exist. Then scripts would break. Now I always check. File tests prevent errors. Use them.