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PHP : Language Reference : Operators : Execution Operators

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.

Code Examples / Notes » language.operators.execution


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

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.

waylanator no

Want to pass a parameter with your batch file to the executable?
Just do this:
@echo off
c:\progra~1\myprog~1\program.exe %1
$par= "my_parameter";
$test=`c:\mybat.bat $par`;
echo "<pre>$test</pre>";
Have more than 1 parameter?  Just add %2 %3 %4 and so on in the batch file.
Hope this helps someone.


Using a backtick script on Linux to make a little directory lister/linker:
 $output = `ls -1|fgrep -v index.php |awk '{ print "<a href=" $1 ">" $1 "</a>" }'`;
 echo "<pre>$output</pre>";
Save as index.php with whatever HTML formatting you desire.
Then, to put this script in the subdirs for recursive listing/linking, do this at the command line:
# find . -mindepth 2 -type d -exec ln index.php {}/ \;
Of course, I'm assuming there is not already a "index.php" file in the subdirs.
A break down of the command line in the backticks:
* ls -1 # list one file per line (note, not "ls -l")
* fgrep -v index.php # exclude the index page
* awk ... # grab each line from ls -1 and wrap with A HREF tags
Have fun!


Or just %* (i think) to pass ALL variables specified


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.


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."

waylanator no

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:
$test = `c:\mybat.bat`;
echo "<pre>$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


might prevent the situation described above from taking place.

shakil dot tanvir

For passing parameter to a executable doesn't need an executable. Also it may create problem specifically for CGI Bin aplicatioin. Have a look at the following code:
$output=`C:\ms4w\Apache\cgi-bin\owtchart.exe $parFile $parImage`;
echo "<pre>$output</pre>";
In the above code "owtchart.exe" takes two parameters. One is a text file(param.txt) and another is a name of a GIF file where output will be created. It works fine and doesn't need any BAT file!


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.


About the french page and french keyboards : backtick is on the 7 key (7 è `), not the english pound key (£ $ ¤).

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