Unix Advanced: Difference between revisions

Advanced Unix Commands

This page is for those who are used to redbrick by now and want to learn how to do more advanced stuff such as running multiple programs at the same time from one terminal window etc.

A process is a program that runs when you execute a command in UNIX. You can have more than one process running at a time, which obviously means you can have more than one program running at a time. "How can this be Conor!?" you shout at me. I cover my ears and respond that you can run some programs in the background or you can put some programs on hold while you do something else equally crazy. And here's how you do all this magic UNIX stuff!

You can put a process on hold so that you can run another program and then come back to it later. Say for example you are reading the newsgroups and someone heys you and you want to reply but you are in the middle of a very deep posting on redbrick.debate. Well all you have to do is press CTRL and Z together on the keyboard and it will "suspend" slrn and bring you back to the shell, now you can hey as normal until you're ready to go back to the newsgroups, in which case you type fg and press enter. This will load up slrn exactly where you left off. Ta da!

Jobs are similar to processes except that jobs are controlled by the shell they're running under, whereas a process could be running on its own without a controlling terminal or shell..(sometimes processes without controlling terminals and in their own process groups are known as "daemons").

Processes forked off (I say "forked off" because the shell program (e.g. bash) normally uses a system function called fork to create a child process which then calls a function named exec to execute whatever program is being run..) by a shell automatically belong to the same process group as the shell, so if the shell exits, so do any other processes in its group since the shell is normally the process to create or own that particular process group id. (This last paragraph has been all an aside so don't worry if you don't have a clue what I'm on about 'cause you're not the only one :o) )

If you suspend a process, or job, as mentioned above, you can start another process while you have a process suspended. You can also have more than one process suspended at a time. This means that when you type fg it will bring up the last program you suspended. However you may want to bring up a job you suspended a while back, which isn't the last job you suspended. (Confusing huh?:o))

In the following example I check what jobs I have running and then bring one of them into the foreground (that's what fg stands for..):

As an aside, the (signal) bit beside mutt means it was suspended by a signal. This is exactly what CTRL Z does. It sends a signal to a program telling it to stop.

Some programs might take a while to run before printing anything, or they may not print anything at all...

That's pretty much all there is to running jobs in the background. You can also explicitly tell a program to run in the background by appending an ampersand to the command, like so:

Don't worry, it's not as evil as it sounds ;o) Sometimes you might not be able to quit a program, or log out properly. A program you're running might have gone into a loop and you mightn't be able to quit out of it, or you might be telnetting or SSHing into redbrick from work or home and you might get logged out because your mam plugged out the phone line or your boss reefed you dossing or something equally unfortunate and when you log back in you see that you're still logged in and you want to log out of that other session.

To kill a particular process just type kill -9 followed by the process number. The -9 means that the process will be closed immediately, it doesn't give it a chance to shut down by itself, it just closes it down as soon as the command is run. In the next example I show the output of ps then I kill one of my processes.. slrn :

As you see above I run ps twice. The first time I run it with the 'u' switch. As you see this gives more detail about processes such as the amount of CPU time and memory that they are taking up.

You see in the above example that when a process gets killed (or finishes itself), the shell will print that the process has finished or has been killed. In the above example you can see that it shows the job number in square brackets, then the reason the process returned (that it was killed) and the name of the process.

Below I will list some of the most common signals and what they're used for. I reserve the right to be very wrong about some of them :o)

CTRL + Z sends.

to write to the terminal. The process gets suspended.

from terminal

will cause the process (if it's a daemon) to re-read its  

configuration files without you having to restart the process  

Sometimes you will see people recommend that you type: kill -9 followed by the process id, to kill a process. This is identical to typing kill -KILL. Indeed if you type the following at the shell :

you will see that SIGKILL is just an alias for the number 9 and the same applies for the other signals (i.e. that they are all aliased to different numbers..)