The exec command specified by the arguments is executed in place of this shell without creating a new process. Input/output arguments may appear and, if no other arguments are given, 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 may be nested, but if they are nested too deeply the shell may run out of file descriptors. An error in a sourced file at any level terminates all nested source commands.

-h

Place commands from the file name on the history list without executing them.

With the exec built-in, if arg is given, the command specified by the arguments is executed in place of this shell without creating a new process. Input/output arguments may appear and affect the current process. If no arguments are given 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:

1. Variable assignment lists preceding the command remain in effect when the command completes.
2. I/O redirections are processed after variable assignments.
3. Errors cause a script that contains them to abort.
4. Words, following a command preceded by ** that are in the format of a variable assignment, are expanded with the same rules as a variable assignment. This means that tilde substitution is performed after the = sign and word splitting and file name generation are not performed.