EXEC(P) POSIX Programmer's Manual EXEC(P)
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 redi-
rections 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 creat-
ing a new process. If arguments are specified, they shall be arguments to command. Redi-
rection affects the current shell execution environment.
See the DESCRIPTION.
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 Consequences of Shell Errors ), the shell shall exit with a value in the range
1-125. Otherwise, exec shall return a zero exit status.
CONSEQUENCES OF ERRORS
The following sections are informative.
Open readfile as file descriptor 3 for reading:
exec 3< readfile
Open writefile as file descriptor 4 for writing:
exec 4> writefile
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:
exec cat maggie
Most historical implementations were not conformant in that:
foo=bar exec cmd
did not pass foo to cmd.
Special Built-In Utilities
Portions of this text are reprinted and reproduced in electronic form from IEEE Std
1003.1, 2003 Edition, Standard for Information Technology -- Portable Operating System
Interface (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2003 by
the Institute of Electrical and Electronics Engineers, Inc and The Open Group. 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 orig-
inal Standard can be obtained online at http://www.opengroup.org/unix/online.html .
IEEE/The Open Group 2003 EXEC(P)