In a Bash script, how can I exit the entire script if a certain condition occurs?

Exiting a Bash Script Gracefully (or Forcefully) on Condition

Learn how to effectively terminate a Bash script based on specific conditions, ensuring proper error handling and predictable behavior using exit codes.

In Bash scripting, controlling the flow of execution is crucial. Often, you'll encounter situations where a script needs to stop immediately if a certain condition isn't met or an error occurs. This article delves into various methods for exiting a Bash script, focusing on conditional exits and the importance of exit codes for communicating success or failure to the calling environment.

Understanding the exit Command

The exit command is the fundamental way to terminate a Bash script. When exit is called, the script stops execution, and control returns to the shell that launched it. The exit command can optionally take an integer argument, known as the exit code or status. This code is a crucial piece of information that indicates the success or failure of the script.

#!/bin/bash

echo "Starting script..."

# Exit with success (0)
exit 0

echo "This line will never be reached."

A simple script demonstrating the basic use of exit 0.

Conditional Exiting with if Statements

The most common way to exit a script based on a condition is by using an if statement. You can check for various conditions, such as the existence of a file, the value of a variable, the success of a command, or the number of arguments provided to the script. If the condition is met (or not met, depending on your logic), the exit command is invoked.

#!/bin/bash

REQUIRED_ARG="hello"

if [ "$1" != "$REQUIRED_ARG" ]; then
  echo "Error: Expected argument 'hello', but got '$1'."
  echo "Usage: $0 hello"
  exit 1
fi

echo "Script executed successfully with argument: $1"
exit 0

Exiting if the first argument does not match a required string.

Workflow for conditional script exits

Exiting on Command Failure

Bash provides powerful mechanisms to automatically exit if a command fails. This is often achieved using set -e or by checking the exit status of individual commands. This approach makes scripts more robust and prevents them from continuing with invalid data or in an incorrect state.

#!/bin/bash

# Option 1: Using set -e (exit immediately if a command exits with a non-zero status)
set -e

echo "Attempting to create a file..."
touch /tmp/my_file.txt

echo "Attempting to remove a non-existent file (will cause exit with set -e)..."
rm /tmp/non_existent_file.txt

# This line will not be reached if rm fails and set -e is active
echo "Script finished successfully."

# Option 2: Checking exit status manually
# rm /tmp/another_non_existent_file.txt
# if [ $? -ne 0 ]; then
#   echo "Error: Command failed."
#   exit 1
# fi

Demonstrating set -e for automatic exit on command failure.

The set -e option is a powerful way to ensure that your script doesn't proceed past a failed command. Alternatively, you can explicitly check the exit status of the last executed command using the special variable $?. This variable holds the exit status of the most recently executed foreground command. A value of 0 indicates success, while any non-zero value indicates failure.

#!/bin/bash

echo "Checking for a non-existent directory..."
ls /non_existent_dir

if [ $? -ne 0 ]; then
  echo "Error: Directory not found. Exiting."
  exit 2
fi

echo "This line will only run if /non_existent_dir existed (it didn't)."
exit 0

Manually checking the exit status of a command.