php-doc-en/functions/exec.sgml

 <reference id="ref.exec">
  <title>Program Execution functions</title>
  <titleabbrev>Program Execution</titleabbrev>

  <refentry id="function.escapeshellcmd">
   <refnamediv>
    <refname>escapeshellcmd</refname>
    <refpurpose>escape shell metacharacters</refpurpose>
   </refnamediv>
   <refsect1>
    <title>Description</title>
    <funcsynopsis>
     <funcdef>string <function>escapeshellcmd</function></funcdef>
     <paramdef>string <parameter>command</parameter></paramdef>
    </funcsynopsis>
    <para>
     <function>EscapeShellCmd</function> escapes any characters in a string that
     might be used to trick a shell command into executing arbitrary commands.
     This function should be used to make sure that any data coming from user
     input is escaped before this data is passed to the <function>exec</function> or <function>system</function>
     functions. A standard use would be:
    <para>
     <informalexample>
      <programlisting>
system(EscapeShellCmd($cmd))
      </programlisting>
     </informalexample>
   </refsect1>
  </refentry>

  <refentry id="function.exec">
   <refnamediv>
    <refname>exec</refname>
    <refpurpose>Execute an external program</refpurpose>
   </refnamediv>
   <refsect1>
    <title>Description</title>
    <funcsynopsis>
     <funcdef>string <function>exec</function></funcdef>
     <paramdef>string <parameter>command</parameter></paramdef>
     <paramdef>string <parameter><optional>array</optional></parameter></paramdef>
     <paramdef>int <parameter><optional>return_var</optional></parameter></paramdef>
    </funcsynopsis>

    <para>
    <function>exec</function> executes the given <parameter>command</parameter>, however it does not
    output anything.  It simply returns the last line from the result of the command.
    If you need to execute a command and have all the data from the command passed
    directly back without any interference, use the <function>PassThru</function>
    function.
    <para>
    If the <parameter>array</parameter> argument is present, then the specified array will be filled
    with every line of output from the command.  Note that if the array already contains some
    elements, <function>exec</function> will append to the end of the array.  If you do not
    want the function to append elements, call <function>unset</function> on the array before
    passing it to <function>exec</function>.
    <para>
    If the <parameter>return_var</parameter> argument is present along with the <parameter>array</parameter>
    argument, then the return status of the executed command will be written to this
    variable.
    <para>
    Note that if you are going to allow data coming from user input to be
    passed to this function, then you should be using <function>EscapeShellCmd</function>
    to make sure that users cannot trick the system into executing arbitrary commands.
    <para>
    See also <function>system</function>, <function>PassThru</function>,
    <function>popen</function> and <function>EscapeShellCmd</function>.
   </refsect1>
  </refentry>

  <refentry id="function.system">
   <refnamediv>
    <refname>system</refname>
    <refpurpose>Execute an external program and display output</refpurpose>
   </refnamediv>
   <refsect1>
    <title>Description</title>
    <funcsynopsis>
     <funcdef>string <function>system</function></funcdef>
     <paramdef>string <parameter>command</parameter></paramdef>
     <paramdef>int <parameter><optional>return_var</optional></parameter></paramdef>
    </funcsynopsis>
    <para>
    <function>System</function> is just like the C version of the function in that it executes
    the given <parameter>command</parameter> and outputs the result.  If a variable is provided as
    the second argument, then the return status code of the executed command
    will be written to this variable.
    <para>
    Note, that if you are going to allow data coming from user input to be passed to this function,
    then you should be using the <function>EscapeShellCmd</function> function
    to make sure that users cannot trick the system into executing arbitrary commands.
    <para>
    The <function>System</function> call also tries to automatically flush the web server's  output
    buffer after each line of output if PHP is running as a server module.
    <para>
    If you need to execute a command and have all the data from the command passed
    directly back without any interference, use the <function>PassThru</function>
    function.  See also the <function>exec</function> and <function>popen</function> functions.
   </refsect1>
  </refentry>

  <refentry id="function.passthru">
   <refnamediv>
    <refname>passthru</refname>
    <refpurpose>Execute an external program and display raw output</refpurpose>
   </refnamediv>
   <refsect1>
    <title>Description</title>
    <funcsynopsis>
     <funcdef>string <function>passthru</function></funcdef>
     <paramdef>string <parameter>command</parameter></paramdef>
     <paramdef>int <parameter><optional>return_var</optional></parameter></paramdef>
    </funcsynopsis>
    <para>
     The <function>passthru</function> function is similar to the
     <function>Exec</function> function in that it executes a
     <parameter>command</parameter>.  If the
     <parameter>return_var</parameter> argument is present, the return
     status of the Unix command will be placed here.  This function
     should be used in place of <function>Exec</function> or
     <function>System</function> when the output from the Unix command
     is binary data which needs to be passed directly back to the
     browser.  A common use for this is to execute something like the
     pbmplus utilities that can output an image stream directly.  By
     setting the content-type to <emphasis>image/gif</emphasis> and
     then calling a pbmplus program to output a gif, you can create
     PHP scripts that output images directly.
    <para>
     See also <function>exec</function> and <function>fpassthru</function>.
   </refsect1>
  </refentry>

 </reference>

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