PHP 7.0.6 Released

    Execution Operators

    PHP supports one execution operator: backticks (``). Note that these are not single-quotes! PHP will attempt to execute the contents of the backticks as a shell command; the output will be returned (i.e., it won't simply be dumped to output; it can be assigned to a variable). Use of the backtick operator is identical to shell_exec().

    <?php
    $output     = `ls -al`;
    echo     "<pre>$output</pre>";
    ?>

    Note:

    The backtick operator is disabled when safe mode is enabled or shell_exec() is disabled.

    Note:

    Unlike some other languages, backticks have no special meaning within double-quoted strings.

    See also the manual section on Program Execution functions, popen() proc_open(), and Using PHP from the commandline.

    User Contributed Notes

    robert
    10 years ago
    Just a general usage note.  I had a very difficult time solving a problem with my script, when I accidentally put one of these backticks at the beginning of a line, like so:

    [lots of code]
    `    $URL = "blah...";
    [more code]

    Since the backtick is right above the tab key, I probably just fat-fingered it while indenting the code.

    What made this so hard to find, was that PHP reported a parse error about 50 or so lines *below* the line containing the backtick.  (There were no other backticks anywhere in my code.)  And the error message was rather cryptic:

    Parse error: parse error, expecting `T_STRING' or `T_VARIABLE' or `T_NUM_STRING' in /blah.php on line 446

    Just something to file away in case you're pulling your hair out trying to find an error that "isn't there."
    cs at kainaw dot com
    10 years ago
    After much trouble, I have concluded that the backtick operator (and shell_exec) have a limited buffer for the return.  My problem was that I was grepping a file with over 500,000 lines, receiving a response with well over 100,000 lines.  After a short pause, I was flooded with errors from grep about the pipe being closed.

    I have searched, but I cannot find the exact size of the buffer used by the backtick operator and shell_exec.  So, to avoid this error, you must limit the output of your commands (such as using -m with grep).  Through trial and error, you can get the command to run without error.
    Anonymous
    3 years ago
    escapeshellarg()

    if you're usimg popen, `` exec or anything else which takes a command-line
    use escapeshellarg() on any variable parts, or risk being pwned.
    vdboor at codingdomain dot com
    10 years ago
    Note that most OS-es define two channels for file-output, the stdout and stderr (standard out and standard error). To read the data sent to stderr too, include 2>&1 in the backticks.
    reed-NO at SPAM-zerohour dot net
    14 years ago
    When a program is run using backticks, and the user cancels page loading (if your program is taking too long!), the shell running the program (the one in the backticks) may continue indefinitely on the server. I do not know if this is a bug, or just a danger of using this feature.  (It may depend on the way the browser "cancels" the request -- it was a problem on both IE and OmniWeb for the Mac).  Beware!
    aaron dot bentley at utoronto dot ca
    12 years ago
    waylanator's example can be dangerous, since it doesn't prevent characters with special meaning from being emitted to the commandline.  Programming errors or untrusted data could cause serious problems.  At the bare minimum, remove all non-alphanumeric characters before passing a string to the shell.  escapeshellarg() is also useful in *nix environments, but usually the best approach is to bypass the shell, using exec() etc.
    inform dot to dot kannan at gmail dot com
    3 years ago
    You can use the backtick operator on windows to run windows command too and it is not restricted only to execute the exe.
    Example:
    <?php
    $output     = `dir`;
    echo     "<pre>$output</pre>";
    ?>
    It lists all the files in the directory
    waylanator no at spam hotmail dot com
    13 years ago
    In Windows it appears you can only call an executable file that resides in the system path which is defined by Windows.  As a workaround you can place a batch file in the system path that calls the program from it's dir. Just make sure to use short MS-DOS file and dir names.
    For example:
    If you were calling the file c:\program files\my program\program.exe do this:

    mybat.bat look like this:
    _________
    @echo off
    c:\progra~1\myprog~1\program.exe
    _________

    Save mybat.bat in c:\ or c:\windows or any other dir in the system path as defined by windows.

    Then in php call the batch file:
    _________
    <?php
    $test     = `c:\mybat.bat`;
    echo     "<pre>$test</pre>";
    ?>
    _________

    That should do it.
    Of course this will only work for a program you can run from the MS-DOS command prompt, but (as I understant it) that goes for any executable you call with PHP anyway.
    Tested in Win98 running Apache 1.3.27 and PHP 4.3.0
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