An exit status is a number that a program passes to the operating system or script once it has finished executing. A value of zero means the program terminated normally, while any non-zero value indicates unexpected termination. The exit status can be used to communicate with other processes and indicate necessary actions. It is a convenient way to pass information without complex communication schemes.
A program’s exit status, also called a return code, is a number that a program passes to the operating system, script, or other process that originally executed it once the program has finished executing. There are no real rules for the meaning of numbers used as the exit status, although a value of zero traditionally means the program terminated normally, and any non-zero value indicates the program terminated unexpectedly. By assigning a number to the exit status, the original program or process that called the code can tell if the program successfully performed the necessary tasks or if other action needs to be taken.
Almost every computer language and many scripting languages have some mechanism or function in place that can return an exit status. There are situations where a program may intentionally terminate in the middle of execution, using a specific error code to indicate that something is wrong, such as a missing data file or a lack of computer memory. Programs that do not explicitly define an exit status code will return a zero to the calling program unless the operating system or language interpreter terminates the program due to errors.
The exit status can also be used to communicate with a script or other process by returning specific values that indicate what action should be taken next. An example would be a script designed to keep a program running despite possible times when the program might go down for maintenance. This scheme would work with the script running the program and then waiting for it to exit. Programmers could organize this so that the program returns a number to indicate that it is exiting permanently and shouldn’t be restarted, and another number could instruct the script to wait a period of time and then restart the program; yet a third value could indicate that the program encountered an error and requires the attention of a programmer.
Using an exit status code for communications is convenient in certain situations due to security and other limitations that some programs have in different operating systems. There are many cases where it is impossible for a program to communicate with its calling process or with another program running parallel on the same system. In these situations, the options for inter-program communications would be limited to complex schemes involving shared memory, network socket connections, or file writing. Using an exit status, simple information can be passed in a very simple way that doesn’t require developing a custom framework.
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