#exec LIST

#exec PROGRAM LIST

The exec function executes a system command and never returns; use system instead of exec 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 of system, Perl warns you if exec is called in void context and if there is a following statement that isn't die, warn, or exit (if warnings are enabled--but you always do that, right?). If you really want to follow an exec with some other statement, you can use one of these styles to avoid the warning:

exec ('foo')   or print STDERR "couldn't exec foo: $!";
{ exec ('foo') }; print STDERR "couldn't exec foo: $!";

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 to execvp, which is more efficient. Examples:

exec '/bin/echo', 'Your arguments are: ', @ARGV;
exec "sort $outfile | uniq";

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:

my $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 or system 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.

my @args = ( "echo surprise" );

exec @args;               # subject to shell escapes
                            # if @args == 1
exec { $args[0] } @args;  # safe even with one-arg list

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 the autoflush method of IO::Handle on any open handles to avoid lost output.

Note that exec will not call your END blocks, nor will it invoke DESTROY methods on your objects.

Portability issues: "exec" in perlport.

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