exec, eval, source - shell built-in functions to execute other commands
source [-h] name
+exec [-c] [-a name] [command [argument ... ]]
The exec command specified by the arguments is executed in place of this shell without creating a new process. Input/output arguments and appear and, if no other arguments are specified, cause the shell input/output to be modified.
The arguments to the eval built-in are read as input to the shell and the resulting command(s) executed.
exec executes command in place of the current shell, which terminates.
eval reads its arguments as input to the shell and executes the resulting command(s). This is usually used to execute commands generated as the result of command or variable substitution.
source reads commands from name. source commands can be nested, but if they are nested too deeply the shell can run out of file descriptors. An error in a sourced file at any level terminates all nested source commands.
With the exec built-in, if arg is specified, the command specified by the arguments is executed in place of this shell without creating a new process. Input/output arguments can appear and affect the current process. If no arguments are specified the effect of this command is to modify file descriptors as prescribed by the input/output redirection list. In this case, any file descriptor numbers greater than 2 that are opened with this mechanism are closed when invoking another program.
The arguments to eval are read as input to the shell and the resulting command(s) executed.
On this man page, ksh(1) commands that are preceded by one or two * (asterisks) are treated specially in the following ways:
exec is a special built-in command that can be used to manipulate file descriptors or to replace the current shell with a new command.
If command is specified, then the current shell process is replaced by command rather than running command and waiting for it to complete. There is no need to use exec to enhance performance since the shell implicitly uses the exec mechanism internally whenever possible.
If no operands are specified, exec can be used to open or close files, or to manipulate file descriptors from 0 to 9 in the current shell environment using the standard redirection mechanism available with all commands. The close-on-exec flags is set on file descriptor numbers greater than 2 that are opened this way so that they are closed when another program is invoked.
Because exec is a special command, any failure causes the script that invokes it to exit. This can be prevented by invoking exec from the command utility.
exec cannot be invoked from a restricted shell to create files or to open a file for writing or appending.
eval is a shell special built-in command that constructs a command by concatenating the arguments together, separating each with a space. The resulting string is taken as input to the shell and evaluated in the current environment. command words are expanded twice, once to construct argument, and again when the shell executes the constructed command. It is not an error if argument is not specified.
On this manual page, ksh93 commands that are preceded by one or two + symbols are special built-in commands and are treated specially in the following ways:
The following options are supported by ksh93 exec:
The following exit values are returned by exec:
The following exit values are returned by exec. If command is specified, exec does not return.
The following exit values are returned by eval:
If argument is not specified, the exit status is 0. Otherwise, it is the exit status of the command defined by the argument operands.
See attributes(5) for descriptions of the following attributes:
csh(1), ksh(1), ksh93(1), sh(1), attributes(5)
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Created 1996-2021 by Maxim Chirkov
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