EXEC(1) User Commands EXEC(1)

NAME

exec, eval, source - shell built-in functions to execute other
commands

SYNOPSIS

sh
exec [argument]...


eval [argument]...


csh
exec command


eval argument...


source [-h] name


ksh
*exec [argument]...


*eval [argument]...


ksh93
+exec [-c] [-a name] [command [argument ... ]]


+eval [argument]...

DESCRIPTION

sh
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.

csh
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.

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


ksh
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:

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.

ksh93
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:

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. They are not valid function names.

5. 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 field
splitting and file name generation are not performed.

OPTIONS

ksh93
The following options are supported by ksh93 exec:

-a name
argv[0] is set to name for command.


-c
Clear all environment variables before executions except
variable assignments that are part of the current exec
command.

EXIT STATUS

ksh
The following exit values are returned by exec:

0
Successful completion.


1-125
A redirection error occurred.


127
command was not found.


126
command was found, but it is not an executable utility.


ksh93
The following exit values are returned by exec. If command is
specified, exec does not return.

0
Successful completion. All I/O redirections were successful.


>0
An error occurred.


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.

ATTRIBUTES

See attributes(7) for descriptions of the following attributes:


+---------------+-----------------+
|ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+---------------+-----------------+

SEE ALSO

csh(1), ksh(1), ksh93(1), sh(1), attributes(7)

April 8, 2008 EXEC(1)