> ## Documentation Index
> Fetch the complete documentation index at: https://notes.kodekloud.com/llms.txt
> Use this file to discover all available pages before exploring further.
# Processing shell command exit codes
> This article explains how exit codes indicate command success or failure in Linux and emphasizes best practices for handling them in scripts.
In this lesson, we explore how exit codes function when executing commands on a Linux system. Every command returns an exit code (also known as a return code) that indicates whether the command executed successfully or encountered an error.
When you run a command like `ls` to list directory contents, it typically returns an exit code of 0 to indicate success:
```bash theme={null}
$ ls
/home /root /tmp
$ echo $?
0
```
In contrast, executing a command that does not exist—such as `lss`—results in an error message and a non-zero exit code:
```bash theme={null}
$ lss
Failed: command not found
$ echo $?
127
```
## Custom Command Example: Rocket Commands
The same principles apply to custom commands. Consider a command named `rocket-status` used to check the status of a rocket launch. This command returns an exit code of 0 if the launch is successful and 1 if it fails:
```bash theme={null}
$ rocket-status
success
$ echo $?
0
$ rocket-status
failed
$ echo $?
1
```
Although exit codes are not displayed in the command output, they are stored in the special variable `$?`. It is important to execute `echo $?` immediately after running the command, as any subsequent command may modify its value.
Always check the exit code immediately after running a command using `echo $?` to ensure you capture the correct value.
## Best Practices for Scripts
It is best practice for scripts—especially those integrated with other scripts—to return appropriate exit codes that signal success or failure. Consider the following example script for launching a rocket:
```bash theme={null}
#!/bin/bash
mission_name=$1
mkdir "$mission_name"
rocket-add "$mission_name"
rocket-start-power "$mission_name"
rocket-internal-power "$mission_name"
rocket-start-sequence "$mission_name"
rocket-start-engine "$mission_name"
rocket-lift-off "$mission_name"
rocket_status=$(rocket-status "$mission_name")
while [ "$rocket_status" == "launching" ]; do
sleep 2
rocket_status=$(rocket-status "$mission_name")
done
if [ "$rocket_status" = "failed" ]; then
rocket-debug "$mission_name"
exit 1
fi
```
In this script:
1. Various setup steps are executed for initiating the mission.
2. The script continuously checks the rocket’s status until it is no longer "launching".
3. If the status changes to "failed", the script runs `rocket-debug` and explicitly exits with an exit code of 1.
Without the `exit 1` command in the failure block, the script would default to an exit code of 0 even when the launch fails, which can lead to improper error handling.
Returning a non-zero exit code upon failure is essential for robust error handling and seamless integration with other scripts. As a best practice, always ensure your scripts return a status code that accurately reflects the outcome of their execution.