Can the Linux “at” command run in environments other than /bin/sh?

Question

I need a scheduler (for one time only actions) for a site I'm coding (in php), and I had two ideas:

1- Run a php script with crontab and verify against a database of scheduled actions and execute ones that are older than current time.

2- Schedule various tasks with the "at" command.

The second option seems much better and simpler, so that's what I'm trying to do. However, I haven't found a way to tell "at" to run a command using the PHP interpreter, and so far I've been creating a .sh script, which contains a single command, which is to run a file through the php interpreter. That is far from the optimal setting, and I wish I could just execute the php code directly through "at", something like:

at -e php -f /path/to/phpscript time

Is it possible? I haven't found anything about using environments other than bash in either the man or online.

Answer 1

You can prepend phpscript with a #!/usr/bin/php (or wherever your php script is stored) and make /path/to/phpscript executable. This is exactly what the #! syntax is for.

Just so it's clear, your phpscript would look like this:

#!/usr/bin/php

...your code goes here

Answer 2

The command you specify to at is executed by /bin/sh , but sh can invoke any command, executed directly or by any specified interpreter.

The following works on my Ubuntu 12.04 system with the bash shell:

$ cat hello.php
#!/usr/bin/php
<?php
    echo "Hello, PHP\n";
?>
$ echo "$PWD/hello.php > hello.php.out" | at 16:11
warning: commands will be executed using /bin/sh
job 4 at Sat Aug 25 16:11:00 2012
$ date
Sat Aug 25 16:11:05 PDT 2012
$ cat hello.php.out
Hello, PHP
$ 

In some cases, you'll have to do some extra work to set environment variables correctly (it's not necessary for this simple case). Quoting the man page:

For both at and batch , commands are read from standard input or the file specified with the -f option and executed. The working directory, the environment (except for the variables BASH_VERSINFO , DISPLAY , EUID , GROUPS , SHELLOPTS , TERM , UID , and _ ) and the umask are retained from the time of invocation.

As at is currently implemented as a setuid program, other environment variables (eg LD_LIBRARY_PATH or LD_PRELOAD ) are also not exported. This may change in the future. As a workaround, set these variables explicitly in your job.