When it comes to writing efficient and reliable shell scripts, one crucial aspect is handling process exit codes. These codes are returned by a command or program after it has executed, and they indicate the success or failure of the operation. As a script writer, it is essential to understand how to handle these exit codes to ensure your script functions as intended. In this article, we will discuss how to exit a shell script based on the process exit code.
Before we dive into the specifics, let's first understand what process exit codes are and how they work. In the Linux/Unix environment, every process has an exit code, which is a numerical value between 0 and 255. A value of 0 typically indicates success, while any other value signifies an error or failure. The exit code is returned to the parent process, which can then use it to determine the outcome of the executed command.
Now, let's say you have a shell script that runs several commands, and you want to exit the script if any of the commands fail. In this case, you would need to check the process exit codes of each command and take appropriate action based on the result. This is where the conditional statement 'if' comes into play.
To check the exit code of a command, you can use the '$?' variable in your script. This variable holds the exit code of the last command executed. So, after each command, you can check the value of '$?' and determine the next course of action. For example, if the exit code is 0, the command was successful, and the script can continue executing. However, if the exit code is any other value, you can use the 'exit' command to terminate the script.
Let's take a practical example. Say you have a script that checks the disk space usage using the 'df' command and sends an email notification if the usage exceeds a certain threshold. The script would look something like this:
#!/bin/bash
df -h | grep /dev/sda1 > /dev/null
if [ $? -ne 0 ]; then
echo "Error in checking disk space"
exit 1
fi
space_usage=$(df -h | grep /dev/sda1 | awk '{print $5}' | sed 's/%//g')
if [ $space_usage -gt 80 ]; then
mail -s "Disk space usage exceeded 80%" admin@example.com
fi
In this script, we first check the exit code of the 'df' command. If it is not 0 (indicating an error), we print an error message and exit the script with a non-zero exit code. Otherwise, we proceed to extract the disk usage percentage and check if it is greater than 80%. If it is, we send an email to the administrator. Notice how we use the 'exit' command to terminate the script if any errors occur.
Another way to handle process exit codes is to use the 'trap' command. This command allows you to define a function to be executed when the script receives a specific signal. In our case, we can use the 'trap' command to execute a function when the script receives a non-zero exit code. Here's an example:
#!/bin/bash
handle_error() {
echo "Error occurred. Exiting..."
exit 1
}
trap handle_error ERR
# rest of the script
In this script, we define a function called 'handle_error' that prints an error message and exits the script. Then, we use the 'trap' command to execute this function whenever the script receives the ERR signal, which is triggered by a non-zero exit code.
In conclusion, handling process exit codes is a crucial aspect of writing reliable shell scripts. By using the 'if' statement or the 'trap' command, you can check the exit codes of commands and take appropriate action based on the result. With this knowledge, you can ensure that your scripts exit gracefully and handle any errors effectively.
