A ‘daemon’ in Linux is program that runs continuously in the background without user intervention processing data.
For instance if we need to parse a log file, or process uploaded videos.
So we write a script and `fork` it separately as an individual process, this can be done in PHP using the Process Control Extensions. Getting a good grip on it, will take some studying. Though the time spent on it would be a valuable if you are in for a lot a heavy duty processing. The pcntl extensions are not available in PHP by default, you have to compile the CGI or CLI version of PHP with –enable-pcntl configuration option when compiling PHP to enable Process Control support.
An another way to run a script in background when pcntl functions are not available to you, would be by writing the script in a unconditional while loop. But make sure you have the logic to stop the process on failure within the loop.
while (1) {
//The complete process
if (condition_to_stop === true) {
break;
}
Then execute this from a control script. Start this a separate process redirecting all the out put to the void.
$processId = shell_exec("nohup /usr/bin/php process.php 1>/dev/null 2>&1 &");
Make sure you do not depend on any output printed in the screen by process.php, as in the above command we send all the output including the error’s into the /dev/null black hole.
Recently I needed to extract the ip number of the box on which my script was running, and had to decide the behaviour of tool.
Upon digging around, worked out a command snippet that worked, as given below.
ifconfig eth0|sed -n "/inet addr:.*/{s/.*inet addr://; s/ .*//; p}"
But the problem was ifconfig only works as a `root` user if you try it as a normal user you would get a command not found.
$ ifconfig
-bash: ifconfig: command not found
The problem is the `sbin` folder under which the `ifconfig` command resides is not added to the`PATH` environment variable by default.
all you have to do is the following.
$ export PATH=$PATH:/sbin
$ echo $PATH
/usr/kerberos/bin:/usr/local/bin:/bin:/usr/bin:/home/raja/bin:/sbin
Now as we have added the `sbin` folder to the `PATH` variable, try the command again it will work.
You don’t want to set the path each time you login. You can add it to your `.bashrc` file in your home folder to update the path
automatically each time you login.
Here is how you do it.,
$ vi ~/.bashrc
add the following line at the end of the file.
export PATH=$PATH:/sbin
Save and close the file. From the next time you log-in the `sbin` folder will be available to your `PATH` variable.
In case you need to reload the .bashrc file immediately issue the following command
$ source ~/.bashrc
NOTE : It works only on a readonly mode. Trying to set the interface parameters as a non root user will generate error.
$ ifconfig eth0 192.168.0.1
SIOCSIFADDR: Permission denied
SIOCSIFFLAGS: Permission denied
