exec
- exec PROGRAM LIST
The
exec
function executes a system command and never returns; usesystem
instead ofexec
if you want it to return. It fails and returns false only if the command does not exist and it is executed directly instead of via your system's command shell (see below).Since it's a common mistake to use
exec
instead ofsystem
, Perl warns you ifexec
is called in void context and if there is a following statement that isn'tdie
,warn
, orexit
(if-w
is set--but you always do that, right?). If you really want to follow anexec
with some other statement, you can use one of these styles to avoid the warning:If there is more than one argument in LIST, this calls execvp(3) with the arguments in LIST. If there is only one element in LIST, the argument is checked for shell metacharacters, and if there are any, the entire argument is passed to the system's command shell for parsing (this is
/bin/sh -c
on Unix platforms, but varies on other platforms). If there are no shell metacharacters in the argument, it is split into words and passed directly toexecvp
, which is more efficient. Examples:If you don't really want to execute the first argument, but want to lie to the program you are executing about its own name, you can specify the program you actually want to run as an "indirect object" (without a comma) in front of the LIST, as in
exec PROGRAM LIST
. (This always forces interpretation of the LIST as a multivalued list, even if there is only a single scalar in the list.) Example:- $shell = '/bin/csh';
- exec $shell '-sh'; # pretend it's a login shell
or, more directly,
- exec {'/bin/csh'} '-sh'; # pretend it's a login shell
When the arguments get executed via the system shell, results are subject to its quirks and capabilities. See `STRING` in perlop for details.
Using an indirect object with
exec
orsystem
is also more secure. This usage (which also works fine with system()) forces interpretation of the arguments as a multivalued list, even if the list had just one argument. That way you're safe from the shell expanding wildcards or splitting up words with whitespace in them.The first version, the one without the indirect object, ran the echo program, passing it
"surprise"
an argument. The second version didn't; it tried to run a program named "echo surprise", didn't find it, and set$?
to a non-zero value indicating failure.On Windows, only the
exec PROGRAM LIST
indirect object syntax will reliably avoid using the shell;exec LIST
, even with more than one element, will fall back to the shell if the first spawn fails.Perl attempts to flush all files opened for output before the exec, but this may not be supported on some platforms (see perlport). To be safe, you may need to set
$|
($AUTOFLUSH in English) or call theautoflush()
method ofIO::Handle
on any open handles to avoid lost output.Note that
exec
will not call yourEND
blocks, nor will it invokeDESTROY
methods on your objects.Portability issues: exec in perlport.