The 5th Annual China PHP Conference

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().

= `ls -al`;


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

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

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User Contributed Notes 8 notes

11 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."
ohcc at 163 dot com
5 months ago
You can use variables within a pair of backticks (``).

= '';
    echo `
ping -n 3 {$host}`;
cs at kainaw dot com
11 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.
4 years ago

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
11 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.
aaron dot bentley at utoronto dot ca
13 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.
reed-NO at SPAM-zerohour dot net
15 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!
waylanator no at spam hotmail dot com
14 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

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:
= `c:\mybat.bat`;

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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