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Sat, Apr 29, 2006 1:24 pm

PHP - Exec

The PHP exec function can be used to call external programs. For instance, if I wish to create a webpage that displays the MD5 checksum for a file, I can call the md5sum program that is present on Unix and Linux systems. If I called the program from a shell prompt on the system, I would see something like the following:

# md5sum file.txt
529dc67dde9486a1af8353915ab94870 file.txt

Using PHP, I can get the MD5 checksum with the following code:

<?php

$filename="mboxgrep-0.7.9-1.i386.rpm";
$md5sum = exec('md5sum '.$filename);
$md5sum = substr($md5sum,0,strpos($md5sum,' '));

?>

The results of the call to the external md5sum program are stored in a variable named md5sum. The md5sum program returns the MD5 checksum followed by a space and then the filename. The filename can be stripped away by using strpos to determine the position of the space in the string and then substr can be used to remove all of the charcters from the string starting with the space to the end of the string.

Since I need to calculate the MD5 checksum, aka hash, regularly, I can create a function that calls the external md5sum program to do so.


function md5sum($filename) {

  $hash = exec('md5sum '.$filename);
  // The md5sum command returns the MD5 hash followed by a space and the
  // filename. Remove the space and filename.
  $hash = substr($hash,0,strpos($hash,' '));
  return($hash);

}

But what if you call an external program that returns multi-line output. If you just store the results obtained by using exec to call the program, you will get only the last line of output for the program.

For instance, I can use the command rpm -qp --requires file.rpm to determine what other software is required by a RPM file. If I call that program with PHP's exec function and assign the results to a variable, requires, however, I get just the last line of the results of calling rpm -qp --requires, which produces multiline output.

<?php
$filename="mboxgrep-0.7.9-1.i386.rpm";
$requires = exec('rpm -qp --requires '.$filename);
?>

What I need to do instead, is put the output of the external command into an array. When using the exec function, I can specify an array to be used to hold the output, by putting a comma after the command to be called and then specifying an array to hold the output of the command.

<?php exec(external_command, $output_array); ?>

For instance, to obtain the output from the rpm command above, I could use the following code:


<?php

$filename="mboxgrep-0.7.9-1.i386.rpm";
exec('rpm -qp --requires '.$filename, $requires);

for ($i = 0; $i < count($requires); $i++) {
   print "$requires[$i]<br>\n";
}

?>

The exec function is used to call the program, storing the output from the rpm command in the array $requires. I can then use a for loop to print each of the lines in the array, putting a <br> tag at the end of each line, so that the HTML output is more readable and matches that of the program. I also use /n to create a new line at the end of each line of output so the source HTML code is more readable, also.

References:

  1. PHP: exec - Manual
  2. MD5
  3. Programming PHP: Chapter 5: Arrays

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