Check for a TTY or interactive terminal in PHP

Fri, Sep 2, 2011 12:42 AM
Many UNIX tools do different things if they are connected to an interactive terminal, also called a TTY. This can be handy for lots of reasons. I had a use case today that prompted me to find out how to do it in PHP.

Here is the situation. We log errors to the PHP error log. We then have processes that monitor that error log and alert us about any uncaught exceptions or fatal errors very quickly so we can address issues. We also monitor non-fatal errors and alert on those on a less frequent schedule. However, this can be annoying if a user is running some code on a terminal that is generating errors. Let's say I am trying to find out why some file import did not happen. Running the job that is supposed to do it may yield an error. Maybe it was a file permission issue or something. There are other people watching the alerts. What they don't know is that I am running the code and looking at these errors in real time. So, they may start digging into the issue when I am the one causing it and can see it happening already. So, I thought it would be nice, if in my error handler, I could not send errors to the error log that are being sent to an interactive terminal. A few quick searches for "php check for tty" did not find anything. In the end, a coworker cracked open a book to see how it was done in C. That got me on the right path to finding two PHP functions: posix_isatty and posix_ttyname. These seem to do the trick. They take a file descriptor like STDOUT and will tell you if that is an interactive terminal and what the tty name is if it is one.

It took me a few tries to get the full effect I wanted in PHP. The thing I always forget is that fatal errors don't use my error handler. I understand why. The engine is in an unknown state when that happens, so it can't keep running code. In the end I added this code to my auto prepend file which is where my error handler, auto loader and other start up stuff is defined for all PHP code we run, both CLI and Apache. Since PHP will send errors to STDERR if it is defined, that is what we want to check for. If it is defined and it is a TTY, we just disable error logging. I should note that I already check the log_errors ini setting in my error handler before I even call the error_log function.
if(defined("STDERR") && posix_isatty(STDERR)){
    ini_set("log_errors", false);
    ini_set("error_log", null);
Hopefully anyone else searching for "php check for tty" or "php interactive terminal" will find this blog post and it will help them out.
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Someone Says:

Doesn't work if you pipe PHP's output:

$ cat testtty.php
    var_dump( posix_ttyname(STDOUT) );

$ php5 testtty.php | cat

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