The exit status or return code of a process in computer programming is a small number passed from a child process (or callee) to a parent process (or caller) when it has finished executing a specific procedure or delegated task. 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 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, which are then ored 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.
Bash shell and script
The exit status of an executed command is the value returned by the waitpid system call or equivalent function. Exit statuses fall between 0 and 255, though, as explained below, the shell may use values above 125 specially. Exit statuses from shell builtins and compound commands are also limited to this range. Under certain circumstances, the shell will use special values to indicate specific failure modes.
For the shell’s purposes, a command which exits with a zero exit status has succeeded. A non-zero exit status indicates failure. This seemingly counter-intuitive scheme is used so there is one well-defined way to indicate success and a variety of ways to indicate various failure modes. When a command terminates on a fatal signal whose number is N, Bash uses the value 128+N as the exit status.
If a command is not found, the child process created to execute it returns a status of 127. If a command is found but is not executable, the return status is 126.
If a command fails because of an error during expansion or redirection, the exit status is greater than zero.
All of the Bash builtins return an exit status of zero if they succeed and a non-zero status on failure, so they may be used by the conditional and list constructs. All builtins return an exit status of 2 to indicate incorrect usage.
The C programming language allows programs exiting or returning from the main function to signal success or failure by returning an integer, or returning the macros
EXIT_FAILURE. On Unix-like systems these are equal to 0 and 1 respectively. A C program may also use the
exit() function specifying the integer status or exit macro as the first parameter.
Apart from the macros
EXIT_FAILURE, the C standard does not define the meaning of return codes. Rules for the use of return codes vary on different platforms (see the platform-specific sections).
In Java, any method can call
System.exit(int status), unless a security manager does not permit it. This will terminate the currently running Java Virtual Machine. "The argument serves as a status code; by convention, a nonzero status code indicates abnormal termination." (
int status is the errorlevel.)
In Linux, Unix and other POSIX-compatible systems the
wait system call sets a status value of type int packed as a bitfield with various types of child termination information. If the child terminated by exiting (as determined by the
WIFEXITED macro; the usual alternative being that it died from an uncaught signal), SUS specifies that the low-order 8 bits of the exit status can be retrieved from the status value using the
WEXITSTATUS macro in wait.h; As such, on POSIX-compatible exit statuses are restricted to values 0-255, the range of an unsigned 8-bit integer.
POSIX-compatible systems typically use a convention of zero for success and non zero for error. Some conventions have developed as to the relative meanings of various error codes; for example GNU recommend that codes with the high bit set be reserved for serious errors, and FreeBSD have documented an extensive set of preferred interpretations. Meanings for 15 status codes 64 through 78 are defined in sysexits.h. These historically derive from sendmail and other Message transfer agents, but they have since found use in many other programs.
In OpenVMS, success is indicated by odd values and failure by even values. The value is a 32 bit integer with sub-fields: control bits, facility number, message number and severity. Severity values are divided between success (Success, Informational) and failure (Warning, Error, Fatal).
Exit codes are directly referenced, for example, by the command line interpreter CMD.exe in the
errorlevel terminology inherited from DOS. .NET Framework processes and the Windows PowerShell refer to it as the
ExitCode property of the
- "Errorlevels". Rob van der Woude's Scripting Pages. Retrieved 2007-08-26.
- "Bash Reference Manual". http://www.gnu.org/. Retrieved 2 September 2013.
- "The GNU C Library Reference Manual 25.6.2: Exit Status". Gnu.org. Retrieved 2012-07-09.
- "Java 1.6.0 API". Sun. Retrieved 2008-05-06.
- The Single UNIX® Specification, Issue 7 from The Open Group – System Interfaces Reference,
- The Single UNIX® Specification, Issue 7 from The Open Group – Base Definitions Reference,
- "Chapter 6. Exit and Exit Status". Faqs.org. Retrieved 2012-07-09.
- FreeBSD Library Functions Manual: preferable exit codes for programs –
- Google search for «"sysexits.h" site:github.com» reports «About 3,540 results»; retrieved 21 Feb 2013 01:30 UTC
- "OpenVMS Format of Return Status Values". H71000.www7.hp.com. Retrieved 2012-07-09.
- "ExitCodes bigger than 255, possible?". Retrieved 2009-09-28.
- "Windows System Error Codes (exit codes)". Retrieved 2009-09-28.
- "MSDN article System Error Codes". Microsoft.