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.
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.
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:exec 5<&0
Close file descriptor 3:exec 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
IEEE Std 1003.1-2001/Cor 1-2002, item XCU/TC1/D6/5 is applied so that the reference page sections use terms as described in the Utility Description Defaults ( Utility Description Defaults ). No change in behavior is intended.