Bash by Example: File Operations

Bash 5.0+

Checking file attributes and types using test operators and conditional expressions, determining if paths exist as files or directories, verifying file permissions and ownership, testing file age and size, and implementing robust file validation logic in shell scripts.

Code

#!/bin/bash

file="example.txt"
dir="data"

# Create dummy files
touch "$file"
mkdir -p "$dir"

# Check if file exists (generic)
if [ -e "$file" ]; then
    echo "$file exists"
fi

# Check if it is a regular file
if [ -f "$file" ]; then
    echo "$file is a regular file"
fi

# Check if it is a directory
if [ -d "$dir" ]; then
    echo "$dir is a directory"
fi

# Check read/write/execute permissions
if [ -r "$file" ]; then echo "Readable"; fi
if [ -w "$file" ]; then echo "Writable"; fi
if [ -x "$file" ]; then echo "Executable"; fi

# Check if file is empty
if [ -s "$file" ]; then
    echo "File is not empty"
else
    echo "File is empty"
fi

# Cleanup
rm "$file"
rmdir "$dir"

Explanation

Bash provides a rich set of unary operators to test file attributes within [ ... ] or [[ ... ]] blocks. These are essential for robust scripts that interact with the filesystem, ensuring files exist before trying to read them or directories exist before trying to enter them.

The most common operators are:

-e: Exists (any type)
-f: Exists and is a regular file
-d: Exists and is a directory
-s: Exists and size is greater than 0

You can also check permissions using -r (readable), -w (writable), and -x (executable). These checks respect the current user's permissions.

Code Breakdown

11

-e is the most basic check. It returns true for files, directories, symlinks, sockets, etc.

16

-f is more specific. It returns false if the path is a directory, even if it exists. This is usually preferred when you intend to read/write content.

31

-s checks if the file has content. It returns true if the file size is > 0 bytes.

Samplebadu

Bash by Example: File Operations

Code

Explanation

Code Breakdown