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perlfunc:exec

by gods
on Aug 24, 1999 at 22:42 UTC ( #196=perlfunc: print w/ replies, xml ) Need Help??

exec

See the current Perl documentation for exec.

Here is our local, out-dated (pre-5.6) version:


exec - abandon this program to run another



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 there is a following statement which isn't die(), warn(), or exit() (if -w is set - but you always do that). 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, or if LIST is an array with more than one value, calls execvp(3) with the arguments in LIST. If there is only one scalar argument or an array with one element in it, 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. Note: exec() and system() do not flush your output buffer, so you may need to set $| to avoid lost output. 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. (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 will be subject to its quirks and capabilities. See `STRING` for details.

Using an indirect object with exec() or system() is also more secure. This usage 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.

    @args = ( "echo surprise" );

    system @args;               # subject to shell escapes
                                # if @args == 1
    system { $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 literally called "echo surprise", didn't find it, and set $? to a non-zero value indicating failure.

Note that exec() will not call your END blocks, nor will it call any DESTROY methods in your objects.


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