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Concept
Exit status
Summary
In computing, the exit status, or exit code, of a terminated process is an integer number that is made available to its parent process (or caller). In DOS, this may be referred to as an errorlevel.
When computer programs are executed, the operating system creates an abstract entity called a process in which the book-keeping for that program is maintained. In multitasking operating systems such as Unix or Linux, new processes can be created by active processes. The process that spawns another is called a parent process, while those created are child processes. Child processes run concurrently with the parent process. The technique of spawning child processes is used to delegate some work to a child process when there is no reason to stop the execution of the parent. When the child finishes executing, it exits by calling the exit system call. This system call facilitates passing the exit status code back to the parent, which can retrieve this value using the wait system call.
The parent and the child can have an understanding about the meaning of the exit statuses. For example, it is common programming practice for a child process to return (exit with) zero to the parent signifying success. Apart from this return value from the child, other information like how the process exited, either normally or by a signal may also be available to the parent process.
The specific set of codes returned is unique to the program that sets it. Typically it indicates success or failure. The value of the code returned by the function or program may indicate a specific cause of failure. On many systems, the higher the value, the more severe the cause of the error. Alternatively, each bit may indicate a different condition, with these being evaluated by the or operator together to give the final value; for example, fsck does this.
Sometimes, if the codes are designed with this purpose in mind, they can be used directly as a branch index upon return to the initiating program to avoid additional tests.
Official source
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