File and Directory Error Handling

Welcome! In this lesson, we will focus on error handling when dealing with files and directories in shell scripts. Managing file and directory existence is a critical part of robust shell scripting. You will learn how to check if files or directories exist and how to verify if scripts have the necessary execution permissions.

Let's get started!

Ensuring a file exists before performing any operations on it can prevent potential errors.

The [ -f $file ] command is used to check if the specified path is a regular file. The -f operator checks if the provided string refers to a regular file (not a directory or other special file types). $file contains the name of the file being checked. This command returns true if $file is a regular file that exists and false otherwise. Consider the following example:

#!/bin/bash

# Creating a file
touch example.txt

# Check if a file exists
file="example.txt"
if [ -f $file ]; then
  echo "File '$file' exists."
else
  echo "File '$file' does not exist."
fi

• touch example.txt creates a file named example.txt in the current directory.
• The variable file is set to example.txt.
• The if [ -f $file ] statement checks if example.txt exists.
• If the file exists, echo "File '$file' exists." prints a confirmation message.
• Otherwise, an alternate message indicates the file does not exist.

The example.txt file exists, so [ -f $file ] returns true and the output is:

File 'example.txt' exists.

Using the -f operator to check if a file exists is crucial to making sure a file exists before performing operations on it.

Similar to checking for a file, we must ensure a directory exists before performing operations on it. Here, we use the -d flag. [ -d $directory ] returns true if the directory at the path stored in $directory exists, and returns false otherwise. Let's see an example:

#!/bin/bash

# Creating a directory
mkdir -p example_dir

# Check if a directory exists
directory="example_dir"
if [ -d $directory ]; then
  echo "Directory '$directory' exists." 
else
  echo "Directory '$directory' does not exist."
fi

In this script:

• mkdir -p example_dir creates a directory named example_dir.
• The variable directory is set to example_dir.
• The if [ -d $directory ] statement checks if example_dir exists.
• If the directory exists, echo "Directory '$directory' exists." prints a confirmation message.
• Otherwise, an alternate message indicates the directory does not exist.

The example_dir directory exists, so [ -d $directory ] returns true and the output is:

Directory 'example_dir' exists.

We use the -d operator to ensure that a directory exists before running commands that require that directory to exist.