This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.
exec — execute commands and open, close, or copy file descriptors
exec [command [argument...]]
The exec utility shall open, close, and/or copy file descriptors as specified by any redirections as part of the command.
If exec is specified without command or arguments, and any file descriptors with numbers greater than 2 are opened with associated redirection statements, it is unspecified whether those file descriptors remain open when the shell invokes another utility. Scripts concerned that child shells could misuse open file descriptors can always close them explicitly, as shown in one of the following examples.
If exec is specified with command, it shall replace the shell with command without creating a new process. If arguments are specified, they shall be arguments to command. Redirection affects the current shell execution environment.
The standard error shall be used only for diagnostic messages.
If command is specified, exec shall not return to the shell; rather, the exit status of the process shall be the exit status of the program implementing command, which overlaid the shell. If command is not found, the exit status shall be 127. If command is found, but it is not an executable utility, the exit status shall be 126. If a redirection error occurs (see Section 2.8.1, Consequences of Shell Errors), the shell shall exit with a value in the range 1−125. Otherwise, exec shall return a zero exit status.
The following sections are informative.
Open readfile as file descriptor 3 for reading:
Open writefile as file descriptor 4 for writing:
Make file descriptor 5 a copy of file descriptor 0:
Close file descriptor 3:
Cat the file maggie by replacing the current shell with the cat utility:
Most historical implementations were not conformant in that:
did not pass foo to cmd.
Section 2.14, Special Built-In Utilities
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 7, Copyright (C) 2013 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) In the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at http://www.unix.org/online.html .
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