How to test file existence in shell script

Introduction

In the world of Linux shell scripting, understanding how to test file existence is a fundamental skill for developers and system administrators. This tutorial provides comprehensive guidance on using various test operators and techniques to verify file presence, permissions, and types in shell scripts, empowering programmers to write more robust and reliable scripts.

Skills Graph

File Existence Basics

What is File Existence?

File existence is a fundamental concept in shell scripting that allows you to check whether a specific file or directory is present in the file system. This is crucial for writing robust and error-handling scripts that can perform different actions based on the presence or absence of files.

Why Check File Existence?

Checking file existence helps developers:

• Prevent errors before performing file operations
• Implement conditional logic in scripts
• Validate input and configuration files
• Ensure script reliability and error handling

Basic File Existence Scenarios

File Types in Linux

Key Considerations

When checking file existence in shell scripts, remember:

• File paths can be absolute or relative
• User permissions affect file accessibility
• Different test conditions apply to various file types

By understanding these basics, you'll be well-prepared to implement file existence checks in your shell scripts on LabEx and other Linux environments.

Shell Test Operators

Test Operator Overview

Shell scripting provides multiple operators to test file existence and characteristics. These operators are essential for conditional file handling in bash scripts.

Primary Test Operators

Syntax Variations

Practical Examples

Basic File Existence Check

#!/bin/bash
FILE="/tmp/example.txt"

if [ -f "$FILE" ]; then
    echo "File exists and is a regular file"
else
    echo "File does not exist"
fi

Multiple Condition Check

#!/bin/bash
FILE="/etc/passwd"

if [ -f "$FILE" ] && [ -r "$FILE" ]; then
    echo "File exists and is readable"
else
    echo "File is missing or not readable"
fi

Advanced Testing

Combining Test Conditions

#!/bin/bash
FILE="/home/labex/script.sh"

if [[ -f "$FILE" && -x "$FILE" ]]; then
    echo "File is a regular executable script"
fi

Best Practices

• Always quote file path variables
• Use [[ for more advanced testing
• Handle potential errors gracefully
• Choose appropriate test operators

By mastering these shell test operators on LabEx, you'll write more robust and reliable shell scripts.

Practical Script Examples

Scenario-Based File Existence Scripts

1. Configuration File Validation

#!/bin/bash
CONFIG_FILE="/etc/myapp/config.conf"

if [ ! -f "$CONFIG_FILE" ]; then
    echo "Error: Configuration file not found!"
    echo "Creating default configuration..."
    mkdir -p /etc/myapp
    touch "$CONFIG_FILE"
fi

2. Backup Script with Existence Check

#!/bin/bash
SOURCE_DIR="/home/labex/documents"
BACKUP_DIR="/backup/documents"

## Ensure source and backup directories exist
if [ ! -d "$SOURCE_DIR" ]; then
    echo "Source directory does not exist!"
    exit 1
fi

if [ ! -d "$BACKUP_DIR" ]; then
    mkdir -p "$BACKUP_DIR"
    echo "Created backup directory"
fi

## Perform backup
cp -r "$SOURCE_DIR"/* "$BACKUP_DIR"

Advanced File Handling Scenarios

3. Log Rotation Script

#!/bin/bash
LOG_FILE="/var/log/myapp.log"
MAX_SIZE=$((10 * 1024 * 1024))  ## 10MB

if [ -f "$LOG_FILE" ]; then
    FILE_SIZE=$(stat -c%s "$LOG_FILE")
    
    if [ "$FILE_SIZE" -gt "$MAX_SIZE" ]; then
        TIMESTAMP=$(date +"%Y%m%d_%H%M%S")
        mv "$LOG_FILE" "$LOG_FILE.$TIMESTAMP"
        touch "$LOG_FILE"
        echo "Log file rotated"
    fi
fi

Error Handling Workflow

Common Use Cases

4. Interactive Script with Multiple Checks

#!/bin/bash
SCRIPT_DIR="/home/labex/scripts"
DEPLOY_SCRIPT="$SCRIPT_DIR/deploy.sh"

## Comprehensive file existence and permission check
if [[ -d "$SCRIPT_DIR" && -f "$DEPLOY_SCRIPT" && -x "$DEPLOY_SCRIPT" ]]; then
    echo "Deployment script is ready"
    "$DEPLOY_SCRIPT"
else
    echo "Deployment setup incomplete"
    echo "Checking: "
    [ -d "$SCRIPT_DIR" ] || echo "- Script directory missing"
    [ -f "$DEPLOY_SCRIPT" ] || echo "- Deployment script not found"
    [ -x "$DEPLOY_SCRIPT" ] || echo "- Deployment script not executable"
fi

Key Takeaways

• Always validate file existence before operations
• Use appropriate test operators
• Implement robust error handling
• Create fallback mechanisms

By practicing these examples on LabEx, you'll develop more reliable and efficient shell scripts.

Summary

By mastering file existence testing in Linux shell scripts, developers can create more resilient and error-handling scripts. The techniques and test operators explored in this tutorial provide a solid foundation for file manipulation, validation, and conditional processing in shell programming environments.