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Programming Perl

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3.2.33 exec

exec LIST

This function terminates the currently running Perl script by executing another program in place of itself. If there is more than one argument in LIST (or if LIST is an array with more than one value) the function calls C's execvp(3) routine with the arguments in LIST. This bypasses any shell processing of the command. If there is only one scalar argument, the argument is checked for shell metacharacters. If metacharacters are found, the entire argument is passed to "/bin/sh -c" for parsing.[3] If there are no metacharacters, the argument is split into words and passed directly to execvp(3) in the interests of efficiency, since this bypasses all the overhead of shell processing. Ordinarily exec never returns - if it does return, it always returns false, and you should check $! to find out what went wrong. Note that exec (and system) do not flush your output buffer, so you may need to enable command buffering by setting $| on one or more filehandles to avoid lost output. This statement runs the echo program to print the current argument list:

[3] Under UNIX, that is. Other operating systems may use other command interpreters.

exec 'echo', 'Your arguments are: ', @ARGV;

This example shows that you can exec a pipeline:

exec "sort $outfile | uniq"
  or die "Can't do sort/uniq: $!\n";

The UNIX execv(3) call provides the ability to tell a program the name it was invoked as. This name might have nothing to do with the name of the program you actually gave the operating system to run. By default, Perl simply replicates the first element of LIST and uses it for both purposes. If, however, you don't really want to execute the first argument of LIST, but you want to lie to the program you are executing about its own name, you can do so. Put the real name of the program you want to run into a variable and then put that variable out in front of the LIST without a comma, kind of like a filehandle for a print statement. (This always forces interpretation of the LIST as a multi-valued list, even if there is only a single scalar in the list.) Then the first element of LIST will be used only to mislead the executing program as to its name. For example:

$shell = '/bin/csh';
exec $shell '-sh', @args;      # pretend it's a login shell
die "Couldn't execute csh: $!\n";

You can also replace the simple scalar holding the program name with a block containing arbitrary code, which simplifies the above example to:

exec {'/bin/csh'} '-sh', @args; # pretend it's a login shell

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